Presently active research programs include diversified and multidisciplinary topics in rarefied gas dynamics, computational fluid dynamics, applied aerodynamics, aircraft icing research, structural dynamics, aeroelasticity, stochastic dynamics, combustion and chemical propulsion, electric propulsion, chemical lasers, optimal orbit analysis, guidance and control, space mechanics, composites, and solar and wind energy. The department promotes a strong interaction with aerospace industries and government agencies which sponsor many of our research projects. The department also maintains a close cooperation in research and education with other departments and research laboratories in the college.
Supercomputer access, departmental workstations, and high-speed networking provide us with new opportunities for computational research activities in various areas including fluid dynamics, structural analyses, performance simulation, space mission control, and optimization of high-energy lasers. Computational Fluid Dynamics Laboratories conduct research in large-scale simulations and technology developments for the solutions of flow phenomena involving aerospace configurations. Dynamics and control of aerospace vehicles are conducted in a separate laboratory. The Aerothermal Simulations Laboratory focuses on fluid dynamic interactions in chemical and nonchemical rocket propulsion chambers. There are also four new thrusts: development of new concepts in nonlinear dynamics; advanced aerospace composites research; experimental aerodynamics for advanced aircraft configurations in our low-turbulence wind tunnel; and computational aerodynamics and solid mechanics.
A Study of Ice Accretion Physics to Improve the Prediction of Ice
Accretion on Aircraft
M. B. Bragg,* S. Lee, H. S. Kim
NASA Lewis Research Center, NAG 3-1988
Aircraft accrete structural ice on forward-facing surfaces when flying
through a cloud of supercooled water droplets. Ice accumulation on the
leading edge of the wing and empenage can cause large losses in the
aerodynamic performance of these surfaces. A better understanding of
the ice accretion physics is sought through experimental and
agricultural studies. In particular, the role played by surface
roughness in the accretion process is critical to the formation of ice
shapes which cause large aerodynamic penalties and is the focus of
much of this research. Also under study is the relationship between
airfoil geometry, ice accretion and the resulting aerodynamic
penalties. This research is part of a NASA program to develop a
predictive capability for the accretion of ice and its effect on
aircraft aerodynamics.
A Natural Low-Frequency Oscillation on Airfoils near Stall
M. B. Bragg,* A. P. Broeren
NASA Lewis Research Center, NGT3-52308
A low-frequency flow oscillation on the upper surface of some airfoils
near stall has been identified. However, the fluid mechanics of this
flow are not completely understood. The periodic flow separation and
reattachment observed is apparently caused by a leading-edge laminar
separation bubble with a Strouhal number of 0.02. Lift oscillations
associated with this flow are quite large and are believed to be
related to stall flutter. Flow visualization, force balance, laser
Doppler velocimetry, and hot-wire studies are being conducted to help
understand this phenomenon.
Effect of Ice Protection Systems on Airfoil Performance
M. B. Bragg,* D. G. Jackson
National Aeronautics and Space Administration, NCA3-AGATE-110
This research is in support of the NASA/FAA/Industry Advanced General
Aviation Transport Experiment program (AGATE) whose goal is to develop
the technology for the next generation of general aviation aircraft.
Research includes the study of the impact of an ice protection system
on the aerodynamic performance of modern laminar flow airfoils.
Research on the effect of residual ice on airfoil performance, and how
to properly scale this phenomenon, is also underway.
Effect of Large-Droplet Ice Accretions on Airfoil and Wing
Aerodynamics and Control
M. B. Bragg,* E. Loth,* S. Lee, T. Dunn, H. Gurbacki
Federal Aviation Administration, 96-G-023
The objective of this research is to study the effects of large-
droplet ice accretions on subsonic aircraft aerodynamics and control.
This type of ice accretion can occur in supercooled large-droplet
icing as well as in smaller droplet clouds at temperatures near
freezing. Wind tunnel tests using simulated ice accretions on an
airfoil with a simple flap will determine the sensitivity to ice size,
shape, and location. Computational fluid dynamics will be used to
extend the results to other airfoil geometries and to higher Reynolds
numbers. Using computational and experimental results, the underlying
fluid mechanics will be studied to help provide a better understanding
of the phenomena.
Aircraft Icing Research Center
M. B. Bragg,* W. R. Perkins,* N. B. Sarter,* H. M. Gurbacki
UIUC Critical Research Initiatives Program
Accidents in all types of aircraft result from the improper use of the
aircraft's ice protection system and the loss of control of the iced
aircraft. This interdisciplinary research program has a goal to
develop the necessary technology for an aircraft to automatically
operate and manage its ice protection, modify its flight envelope to
avoid maneuvers where flight could potentially be uncontrollable, and
if necessary, adapt the control system. A human-centered approach
would ensure proper pilot-automation coordination. For additional
safety, exit strategies would be developed for severe conditions where
envelope protection was not sufficient.
Critical Ice Accretion on Aircraft Wings
M. S. Selig,* M. B. Bragg,* F. Saeed
NASA Lewis Research Center, NCC3-509
Aircraft wing ice accretion depends on several factors including
liquid water content, droplet size, and temperature. The most
important, however, is the airfoil leading-edge geometry where the ice
first accretes. Owing to myriad scaling issues, full-scale tests are
highly desirable, but the costs are often prohibitive. An approach has
been devised that has the advantages of full-scale tests without the
associated costs. The full-scale airfoil leading edge is tested along
with a foreshortened aft section. The approach will be validated
through tests at NASA Lewis on a "hybrid" airfoil based on
the Learjet Model 45 main-wing airfoil. Extension of this work to
three dimensions is planned.
Development of a Turbulence Model for Unmanned Aerial Vehicles
M. S. Selig,* C. P. Ninham, R. Raju
U.S. Naval Command Control and Ocean Surveillance Center
The development of a flight simulator for navy unmanned aerial vehicle
pilot training requires a model for the turbulent atmospheric
environment encountered at sea. The current approach employs a three-
dimensional Monte Carlo turbulence simulation code (SNLWIND-3D). The
code correctly simulates the point-to-point correlation of 3-D gusts,
both laterally and longitudinally. Thus, effects such as longitudinal
swirl, which can cause the aircraft to roll abruptly, are being
incorporated into the turbulence model. Integration of the model into
the flight simulator has led to a more realistic simulation of the 3-D
turbulence around the ship.
Inverse Design of Complex Wings and Helicopter Rotor Blades
M. S. Selig,* A. Gopalarathnam
Boeing Corp.; McDonnell Aircraft and Missile Systems
Traditionally, airfoils are designed in isolation using two-
dimensional design methods even though they typically operate in a
three-dimensional flow field. Whether or not the 3-D design
requirements are achieved is determined by post-design analysis of the
wing or rotor blade. To satisfy the design requirements, the designer
cycles between the airfoil-design code and the wing-analysis method.
This cut-and-try approach is tedious and time consuming. The current
research aims to develop inverse-design tools that allow the designer
to prescribe the desired flow characteristics over the wing as inputs
and determine the shapes of the wing that satisfies these
prescriptions.
Viscous Inverse Design of Multielement Airfoils
M. S. Selig,* A. Gopalarathnam
Ford Motor Co.
High-lift multielement aifoils find application not only to aircraft
wings for shortening takeoff and landing distances but also to racecar
wings for increasing cornering speeds. A design method has already
been developed under the current research program to design
multielement airfoils with desired inviscid behavior. The method uses
a novel hybrid approach by coupling an isolated-airfoil design method
to generate the individual elements and a panel method to analyze the
multielement airfoil. A key feature of the method is the inexpensive
computation of sensitivities, enabling rapid interactive design. Work
is currently under way to extend the method for viscous inverse
design.
A CFD Development and Application Project Focused on IndyCar Road
Course Front Wings
M. S. Selig,* A. Filippone
Newman Haas Racing
The objective of this research is to develop a reliable CFD capability
for use in indy car front wing design. The approach will be to move in
successive steps from the rather simple (yet still computationally
difficult) 2-D wing in-ground-effect case to finally 3-D complex
IndyCar front wings, including the nose cone and wheels. Wind tunnel
tests will be performed to validate the CFD tools. This work will
culminate in the application of the methodology to the design of wind
tunnel test pieces to influence the design of the Swift/NHR chassis.
Follow-on work might involve a study of the transient effects of the
car bouncing over bumps.
Horizontal Axis Wind Turbine Blade Design
M. S. Selig,* P. Giguère
National Renewable Energy Laboratory, XAF-4-14076-03
A computer program called BladeOpt is being developed to facilitate
the aerodynamic blade design of horizontal axis wind turbines (HAWTs).
More precisely, BladeOpt is a genetic algorithm-based optimization
method that uses the PROPID code for the function evaluations. Given a
set of design requirements/constraints and bounds for the parameters
to be optimized, BladeOpt provides optimum blade geometries for energy
capture. In addition, BladeOpt will also have a multioptimization
capability that combines aerodynamic and structural design
considerations. This new design method is an improvement over the
traditional design by analysis method as it can significantly reduce
the cycle time of the blade design process while increasing the
performance of HAWTs.
Airfoil Design for Small Horizontal Axis Wind Turbines
M. S. Selig,* P. Giguère
National Renewable Energy Laboratory, XAF-4-14076-03; WindLite Co.
Small horizontal axis wind turbines (HAWTs) present an attractive
alternative source of electricity especially in remote areas. For
small HAWTs, the rotor blades usually operate at low Reynolds numbers
along the entire span. The energy capture of small HAWTs can be
severely degraded if the selected airfoils are not suitable for this
flow regime. In an effort to enhance the performance of small wind
energy systems, specially tailored airfoils for small HAWTs were
designed and wind tunnel tested. The airfoils cover a broad range of
operating conditions and are suited for both conventional (tapered
and/or twisted) and pultruded blades.
Influence of Leading-Edge Tape on the Aerodynamic Performance of Wind
Turbines
M. S. Selig,* P. Giguère
National Renewable Energy Laboratory, XAF-4-14076-03
During typical operations, wind turbines are exposed to various
abrasive airborne particles that erode the blade's leading edge. Over
time, these airborne particles can significantly damage the leading
edge and thus reduce aerodynamic performance. The application of
leading-edge tape is a widely used low-cost method to protect the
leading edge of wind turbine blades against erosion. Even though
leading-edge tape is commonly used, especially on small wind turbines,
its effect on aerodynamic performance and application strategies has
not been investigated. In an effort to minimize the energy loss of
wind turbines using leading-edge tape, a systematic study was
performed to quantify its effect and provide guidelines for optimum
application.
Design of High-Lift Multielement Airfoils for Race Car Wings
M. S. Selig,* A. Gopalarathnam, W. J. Jasinski, A. Filippone
Ford Motor Co.
High aerodynamic down force generated by the wings of race cars is
crucial to maximizing cornering performance. Currently, design tools
are being developed for high-lift airfoils to be used on the wings of
open-wheel race cars. These airfoils will maximize the down force
while satisfying geometry constraints imposed by the race rules. An
inverse design method for multielement airfoils and a Matlab-based
graphical user interface provide a rapid, interactive design
environment. Wind-tunnel tests and computational analysis using VSAERO
and PMARC will be performed to develop a front wing/underbody
aerodynamic model, which will be used to enhance the design tools.
Horizontal Axis Wind Turbine Performance Prediction/Model Development
M. S. Selig*
National Renewable Energy Laboratory, XAF-4-14076-03
Noticeable discrepancies exist between wind turbine field test data
and predicted power output from the blade element/momentum methods.
Power is typically underpredicted at high wind speeds and
overpredicted at low wind speeds. These discrepancies can be
attributed to induced effects that are not properly accounted for by
the classical Prandtl tip-loss model. A more accurate and
computationally efficient tip-loss model will be developed based on
results from two state-of-the-art vortex-method rotor codes and recent
field test data. The new model will then be integrated into existing
performance prediction methods used in design.
An Improved Prescribed Wake Analysis for Wind Turbine Rotors
M. S. Selig,* C. J. Fisichella
University of Illinois
In an effort to improve the performance modeling capabilities of wind
turbine rotor analysis codes, an experimental program is being
conducted to obtain wind turbine wake measurements. Model rotors will
be tested in the UIUC 5 x 5 ft low-speed wind tunnel. Schlieren flow
visualization will be the primary method used to obtain wake geometry
measurements. Hot wire anemometry will be used to obtain point
velocity measurements of the wake region immediately behind the rotor
blades. An improved wake model will be developed from the data
collected. This model will be incorporated into an existing rotor
analysis code.
Horizontal Axis Wind Turbine Post-Stall Performance Prediction--
Experimental Data Investigations
M. S. Selig,* C. J. Fisichella, J. Whale, Z. Du
National Renewable Energy Laboratory, XCX-7-16466-01
The first goal of this research is to acquire, interpret, and suitably
reduce all available, high-quality experimental wind turbine rotor
test data for use in a three-dimensional rotor post-stall modeling
effort. The second is to develop an improved technique for determining
the equivalent two-dimensional airfoil data from these field data. A
prescribed wake code is used in an iterative fashion with the measured
pressure data to obtain an estimate of the equivalent airfoil section
data. These data will be used to better predict wind turbine post-
stall performance.
Low Reynolds Number Airfoil Design and Wind Tunnel Testing
M. S. Selig,* C. A. Lyon, P. Giguère, A. P. Broeren, A.
Gopalarathnam, C. A. Carroll
AeroVironment, Inc.; Private gifts
This research deals with enhancing the performance of airfoils for
operation at low Reynolds numbers. For such airfoils, boundary-layer
transition takes place through a laminar separation bubble that forms
as the laminar boundary layer first separates, then becomes unstable,
transitions to turbulent flow and reattaches to the airfoil to form
the bubble. High drag produced by the bubble is the principal cause
for the performance degradation at low Reynolds numbers. Wind tunnel
tests are being performed to validate newly developed low Reynolds
number airfoil design philosophies aimed at mitigating the adverse
bubble effects.
Effects of Boundary Layer Trips on Laminar Separation Bubbles
M. S. Selig,* C. A. Lyon
University of Illinois; Private gifts
As a consequence of laminar separation bubbles, low Reynolds number
airfoils often develop high drag and nonlinear lift curves. In an
effort to prevent these bubbles from forming, and subsequently to
improve airfoil performance, boundary-layer trips are used to promote
early transition. Unfortunately, a poor understanding of transition
enhancement by means of boundary-layer trips makes trip optimization
difficult. Therefore, as an aid in determining what parameters have
the largest influence on trip performance, extensive performance and
flow visualization data will be taken on airfoils that exhibit laminar
separation bubbles. These results will be compared with data taken on
the same airfoils but with varying trip geometries and locations.
Design of High-Lift Multielement Airfoils for Race Car Wings
M. S. Selig,* A. Gopalarathnam, W. J. Jasinski, A. Filippone
Ford Motor Co.
High aerodynamic down force, generated by the wings of race cars, is
crucial to maximizing cornering performance. Currently, design tools
are being developed for high-lift airfoils to be used on the wings of
open-wheel race cars. These airfoils will maximize the down force
while satisfying geometry constraints imposed by the race rules. An
inverse design method for multielement airfoils and a Matlab-based
graphical user interface provides a rapid, interactive design
environment. Wind-tunnel tests and computational analysis using VSAERO
and PMARC will be performed to develop a front wing/underbody
aerodynamic model, which will be used to refine the design tools.
Slot-Suction Airfoils for Flying Wing Configurations
M. S. Selig,* F. Saeed, Z. Du
University of Illinois
With the advent of advanced technologies, the aerospace industry is
currently exploring unconventional configurations for transport
airplanes capable of carrying 600 to 800 passengers. Studies suggest a
flying-wing configuration for any greater-than-600-passenger airplane.
Since the wings for such aircraft are thick, boundary layer control
(BLC) is advantageous. In an effort to aid in the design and testing
of thick boundary layer control wings, a design methodology for slot-
suction airfoils was successfully developed at the University of
Illinois. The goal of current research is to design practical airfoils
with suction BLC for use in preliminary design studies.
Development of a Reconfigurable Aircraft Flight Simulation Laboratory
K. R. Sivier,* R. Oltman
University of Illinois
An aircraft simulator (which was obtained with support from NSF, the
UIUC College of Engineering, and Frasca International) is being
integrated into a Flight Simulation Laboratory for use in the AAE
undergraduate program. Currently, the work is focused on developing
instructional models for use in AAE undergraduate courses, modifying
the propulsion system models, building databases for both aircraft and
propulsion systems, integrating an external control system simulation
module into the simulator system, and validating the simulator
performance.
Optimal Low-Thrust Interception and Deflection of Earth-crossing
Asteriods
B. A. Conway*
University of Illinois
A consensus is developing that while the probability of a large
asteroid or comet colliding with the Earth is low the potential for
destruction is immense, and thus some resources should be devoted to
threat detection and possible interdiction. In this work, optimal
trajectories are determined for the interception of asteroids which
pose a threat of collision with the Earth. An impulsive-thrust escape
from the Earth is used initially to reduce flight time but is followed
with continuous low-thrust propulsion because of the significant
propellant mass advantages of electric propulsion. The objective is to
maximize the eventual closest approach distance for a given small
impulse applied to the asteroid, perhaps by the explosion of a nuclear
weapon.
Optimal Control for Atmospheric Reentry
B. A. Conway,* P. N. Desai
University of Illinois; NASA Langley Research Center
The problem of optimal control for atmospheric reentry is examined
with the objective of minimizing the number and size of the
controllers. For example, the trajectory of a future Space Shuttle-
type vehicle might be determined in such a way as to simultaneously
mimimize the fuel required for reentry and the size of the flight
control surfaces such as ailerons, elevators, and rudder needed for
the vehicle to maneuver in the atmosphere. The DCNLP (direct
collocation with nonlinear programming) method will be used to convert
the continuous problem into a discrete problem.
Improvement of Differential Inclusion Solutions to Optimization
Problems
B. A. Conway,* K. M. Yepez
University of Illinois
Various methods exist for discretizing a continuous optimization
problem and converting it into a nonlinear programming (NLP) problem.
Improvements to these methods are of interest because there is a
practical upper limit to the size of the NLP problem which can be
solved. The differential inclusion method is useful in some cases
because control variables do not appear explicitly, reducing the
number of NLP variables in a given problem. In this work, the implicit
numerical integration formulas that are used with the differential
inclusion method will be replaced with higher-order formulas so that
the discretization may be accomplished with a coarser mesh and achieve
the same accuracy. The robustness and speed of the solutions will be
compared to those obtained using extant methods.
Optimal Aero-assisted Orbit Interception and Rendezvous
B. A. Conway,* K. Horie
University of Illinois; Japan Defense Agency
Optimal, minimum-fuel or minimum-time trajectories are found for the
interception or rendezvous of one spacecraft with another. The
intercepting spacecraft may use impulsive velocity changes, provided
by a rocket motor, and may also travel through the upper region of the
Earth's atmosphere and use atmospheric drag and lift to change its
orbit, in order to accomplish the interception. Orbit change which is
accomplished by aerodynamic forces thus supplements the propulsive
orbit change and reduces total fuel use. When passing through the
atmosphere the vehicle experiences intense aerodynamic heating;
practical heating rate constraints thus constrain the feasible
trajectories.
Automated Docking of Microsatellites
V. Coverstone-Carroll,* C. Hammill
University of Illinois (In conjunction with the Advanced Digital
Signals Lab in the Department of Electrical and Computer Engineering)
Use of spacecraft in the 50- to 100-kg range, launched as secondary
payloads, has enabled nontraditional users to take advantage of space
resources. Future missions may require the docking of microsats for
replenishment of mission consumables or the inclusion of a secondary
structure such as an antenna. Docking mechanisms for large spacecraft
historically include avionics packages weighing in the hundreds of
kilograms. Mass and size restrictions on microsats necessitate a novel
approach. An automated docking scheme adhering to these restrictions
is being developed and ground tested.
Detumbling and Reorienting an Underactuated Rigid Spacecraft
V. Coverstone-Carroll*
University of Illinois
Control algorithms that detumble and reorient underactuated spacecraft
are investigated. An underactuated spacecraft is defined as having
control torques supplied by external thrusters about only two of the
principal axes. Variable structure control is used to detumble the
spacecraft.
Trajectory Optimization of a Solar Sail Spacecraft in a Three-Body
System
V. Coverstone-Carroll,* B. Powers
University of Illinois
Propulsion of a spacecraft using a solar sail is an old concept that
has recently gained popularity due to an industry drive for a cost-
effective propulsion system. Most of the research to date has studied
the effect of a solar sail on the position and stability of the
equilibrium points in three-body systems such as the Earth-Sun
spacecraft or Earth-Moon spacecraft. This research optimizes a variety
of trajectories for a solar sail in a three-body system. Total flight
time, solar array size, and payload capacity are varied to establish a
trade space for mission-planning purposes.
Propellant-minimizing Orbit Selection Subject to Environmental
Disturbances
V. Coverstone-Carroll,* G. Rogers
University of Illinois
The effect of environmental disturbance forces and torques
(atmospheric drag, gravity gradient, solar radiation, and magnetic
torque) on a spacecraft is analyzed to select propellant-minimizing
orbits. The total propellant required to obtain, maintain, and deorbit
from a specified orbit is considered. Solid modeling programs such as
Unigraphics aid in approximating the torques applied to the spacecraft
due to the space environment. Practical applications for existing and
proposed satellite constellations are being examined.
Simulated Annealing and Genetic Algorithms for Generation of Near
Optimal Low-Thrust Trajectories
V. Coverstone-Carroll,* J. Hartmann
University of Illinois; Jet Propulsion Laboratory
The heuristic optimization techniques of simulated annealing and
genetic algorithms are used to generate near-optimal low-thrust
solutions. Trajectories are discretized for a given number of segments
and the thrust direction is optimized over each segment. Results of
the two methods are compared and contrasted for accuracy and
robustness. Individual advantages are noted for the purpose of
producing a hybrid algorithm that combines the benefits of each
individual method.
Trajectory Modeling and Optimization for Future Space Systems
V. Coverstone-Carroll,* C. Hartman
University of Illinois; Spectrum Astro Inc.; U.S. Air Force Phillips
Laboratory
In an effort to minimize the cost of deploying and operating space
systems and to increase competitiveness, mission planners are
examining a variety of new technologies for boosting space systems
into orbit and for performing propulsive maneuvers to achieve and
maintain mission orbits. In addition, spacecraft builders are
producing smaller bus designs to allow for the use of smaller launch
vehicles and for multiple launches aboard a single launch vehicle.
Research into developing mathematical and computational modeling and
optimization techniques to effectively perform the needed mission
analysis and operational planning is being performed.
Optimal Interplanetary Spacecraft Trajectories Via a Pareto Genetic
Algorithm
V. Coverstone-Carroll,* J. Hartmann, W. Mason
University of Illinois; NASA Jet Propulsion Laboratory
A Pareto genetic algorithm is applied to the optimization of low-
thrust interplanetary spacecraft trajectories. A multiobjective,
nondominated sorting algorithm is developed following existing
methodologies. Verification of expected performance is accomplished
through application of the algorithm to a suite of multiobjective test
functions. A hybridized scheme is designed which integrates the Pareto
genetic algorithm with a calculus-of-variations-based trajectory
optimization algorithm. "Families" of Pareto optimal
trajectories are generated for the cases of Earth-Mars flyby, Earth-
Mars rendezvous, and Earth-Mercury rendezvous trajectories.
Application of Parallel Recombinative Simulated Annealing to
Propellant Minimizing Low-Thrust Trajectories
V. Coverstone-Carroll,* J. Domercant
University of Illinois
Parallel recombinative simulated annealing is a hybrid computer
algorithm combining both simulated annealing and genetic algorithm
concepts and attempts to incorporate and augment the traditional
strengths of each while simultaneously eliminating their respective
weaknesses. This optimization technique is applied to interplanetary
missions. Trade studies involving minimum transfer time and maximum
delivered mass are being performed. Preliminary results show that
parallel recombinative simulated annealing combines the speed and
parallel nature of genetic algorithms with the probabilistic hill-
climbing ability, selectivity, and guaranteed convergence of simulated
annealing.
Optimal Constellation Design through Genetic Algorithms
V. Coverstone-Carroll,* W. Mason
University of Illinois
Modern satellite systems have started using large constellations;
however, little investigation has been performed to optimize their
configuration. Historically, continuous global coverage has been the
only consideration, but launch cost, manufacturing difficulty, and
payload and communications performance are all issues directly
affected by the choice of orbital configuration. A general tool for
constellation design optimization is being developed which includes
software that uses a genetic algorithm with flexible cost
considerations and indicators of merit.
Second-Order Necessary and Sufficient Conditions for Optimal Control
Problems
J. E. Prussing,* J.-W. Jo
University of Illinois
A procedure is developed to apply second-order necessary and
sufficient conditions for a weak local minimum of the Bolza optimal
control problem. For a system with matrix (indicating a conjugate
point) into a test for a scalar being zero. Examples include a minimum
time problem, an optimal spacecraft trajectory, minimum distance on a
sphere, and Hamilton's principle. When an extremal solution violates
the second-order necessary conditions, a genetic algorithm is used to
obtain the optimal solution.
Constant Radial Thrust Acceleration Redux
J. E. Prussing,* V. Coverstone-Carroll*
University of Illinois
In 1953 Tsien analyzed escape from a circular orbit in the two-body
problem using a constant radial thrust acceleration. This problem is
revisited, providing more analytical results and a novel application.
The concept of a potential well is used to determine under what
conditions escape will occur and a closed-form expression for the
amplitude of the radial oscillation if escape does not occur. In
addition, the concept of using a constant radial thrust acceleration
to shift the radius of a circular orbit of specified period (or to
shift the period for a specified radius) is introduced and analyzed.
Stability of Nonpremixed Flame-Sheets Held by a Splitter Plate
J. Buckmaster,* R. Weber (Univ. of New South Wales)
National Aeronautics and Space Administration, NAG 3-1704; U.S. Air
Force Office of Scientific Research, F49620-96-1-0031
A stream of air and a stream of fuel separated by a semi-infinite
splitter plate mix downstream of the plate trailing edge and can
support a flame. This flame consists of a stream-wise sheet, a
diffusion flame, and an edge structure lying off the plate edge where
premixing occurs. A simple model is being used to describe the steady
solution and its stability. There is a minimum Damkohler number below
which the flame is blown off. Experiment suggests that, for some
mixtures, pulsating instabilities will precede blow-off, and this is
being explored theoretically.
Detonation Waves in Dusty Gases
J. Buckmaster,* C. Lewis
U.S. Air Force Office of Scientific Research, F49620-96-1-0031
This project is concerned with the effects of dust, inert or reactive,
on the structure of a 1-D detonation wave. The particles are large
enough that the temperatures and the velocities of the two phases are
different, and this can lead to quenching. An essential ingredient of
the steady structure is the location of the sonic point in the region
of nonvanishing reaction, a situation different from the classical
Chapman-Jouget detonation. A numerical strategy and an asymptotic
strategy are being used.
Minimum Energies for Detonation Initiation in Dusty Gases
J. Buckmaster,* C. Lewis
U.S. Air Force Office of Scientific Research, F49620-96-1-0031
Deposition of energy in an explosive mixture (by a spark, for example)
will only lead to an explosion if the energy is sufficient. Knowledge
of the sufficiency is important for safety reasons. A combined
numerical/asymptotic strategy is being used to calculate minimum
initiation energies for explosive mixtures that contain substantial
amounts of dust.
Edge-Flames in Premixed Combustion
J. Buckmaster,* T. G. Vedarajan
U.S. Air Force Office of Scientific Research, F49620-96-1-0031;
National Aeronautics and Space Administration, NAG 3-1704
This study is concerned with premixed flame-sheets with edges, edges
that either propagate forward as ignition waves or retreat as failure
waves. They can arise when the flame configuration has two stable
solutions (bistable equilibrium) and this occurs when a premixed flame
is subject to a straining flow, or when it is subject to radiative
heat losses. The ramifications for turbulent premixed combustion in
the laminar flamelet regime are being explored.
Flame-Ball Drift
J. Buckmaster,* P. Ronney (Univ. of Southern California)
National Aeronautics and Space Administration, NAG 3-1704
During the STS-83 and STS-94 space shuttle missions of summer 1997, it
was observed that flame-balls tend to drift relative to one another at
a speed on the order of 1 cm/min. A theory of flame-ball drift is
being developed. One driving mechanism is a temperature gradient, and
a second is a gradient of fuel concentration.
Radiation Effects on Stretched Flames
J. Buckmaster,* K. Maruta (Tohoku Univ.)
National Aeronautics and Space Administration, NAG 3-1704
Recent Japanese drop-tower experiments on a deflagration in a simple
counterflow have shown that, absent gravitational distortions,
radiation losses can have a significant effect for weak mixtures. A
simple model is able to describe the remarkably complex response of
the flame to different stretch rates and predicts combustion limits
consistent with the experimental record. However, these limits bear no
relation to the fundamental intrinsic flammability limit for an
unstretched flame.
Flame Balls in Wet Mixtures
J. Buckmaster,* M. Smooke (Yale Univ.)
National Aeronautics and Space Administration, NAG 3-1704
Flame balls are stationary, spherical, premixed flames, observed under
microgravity conditions, in which there are no advective fluxes. They
are studied in order to achieve fundamental insights into flammability
limits. Limits arise because of radiation losses, and these can be
changed by adding water-vapor to the mixture. Modeling enables us to
explore the nature of these changes and so provide a guide for future
experiments, designed by P. Ronney (University of Southern California)
to be performed onboard the space shuttle.
Flame Spread
J. Buckmaster,* T. G. Vedarajan
National Aeronautics and Space Administration, NAG 3-1704
Flame spread over a solid or liquid fuel bed is strongly influenced by
the details of the combustion field near the leading edge and by the
physical interaction between this edge and the bed. Models are being
constructed which, it is hoped, will provide simple insights into the
nature of these effects.
Convergence Acceleration for Navier-Stokes Computations
K. D. Lee,* S. Eyi
McDonnell Douglas Corp.
The objective is to achieve a high rate of convergence in the solution
procedure for the Euler and Navier-Stokes equations in high-speed flow
calculations. Many existing acceleration schemes perform well for
linear problems, but deteriorate as the degree of nonlinearity
increases. Their performance depends on flow condition, grid size, and
grid quality. The efficiency and robustness of convergence is improved
through a combined use of generalized minimal residual (GMRES),
implicit preconditioner, and multigrid in transonic flow calculations.
Further study includes schemes such as vector-sequence extrapolation,
incomplete LU factorization, and a multidimensional preconditioner.
Grid Adaptation to Geometry and Flow Solution
K. D. Lee,* K. K. Chand
NASA Lewis Research Center, NAG 3-1148
The accuracy of a flow solution can be greatly enhanced by using a
grid tailored to the specific geometry and flow solution. The
objective is to investigate the need for quality grids and to develop
a grid adaptation technique that improves grid quality in a systematic
and automatic manner. A grid adaptation scheme is developed based on
the potential theory and numerical mapping. In the approach, a
distribution of grid control sources is defined from a flow solution
and/or a grid quality measure of an initial grid, and grid points are
redistributed through the influences of the grid control source. The
use of adapted grids demonstrates a significant improvement in the
quality of resulting flow solutions.
Shape Optimization for Transonic Compressors Based on Navier-Stokes
Physics
K. D. Lee,* K. K. Chand, J. Chung
NASA Lewis Research Center, NAG 3-1983
The objective of this research is to develop an automated CFD-based
shape optimization tool for high-speed compressor designs. It will be
based on Navier-Stokes physics with proper turbulence modeling and
computational grids to produce reliable designs at operation
conditions. It is to provide an inverse capability to find geometries
that produce improved performance at various operating ranges.
Constraints will be imposed to prevent downgrading of other
performance characteristics while enforcing design objectives.
Aerodynamic Design Optimization Using Parallel Computing
K. D. Lee,* M. Damodaran
University of Illinois
We developed an aerodynamic design technique that combines CFD
analysis with numerical optimization. It is based on high-level flow
modeling to improve the reliability of designs. The Navier-Stokes
equations are solved on refined grids to simulate the flowfield
accurately. This implies that the design process requires large memory
and long computing time. Those requirements are the primary factors
that limit practical applications because of the high equipment and
computing cost and slow turn-around time. A solution to this problem
is the use of parallel computing. In this project, therefore, our
design code will be ported to the parallel computing environment, and
a parametric study will be conducted to improve the performance of
parallel computing.
Design Optimization of Multielement Airfoils
K. D. Lee,* S. Eyi
NASA Ames Research Center, NCA 2-833
High-lift aerodynamics is one of the items pending in the development
of future transport aircraft. The current trend is toward a more
efficient and simpler design to reduce weight and cost. The objective
is to produce a reliable and affordable design code for multielement
high-lift airfoils that finds geometries of improved aerodynamic
performance with increased lift and reduced drag at take-off and
landing configurations. It uses an incompressible Navier-Stokes flow
solver (INS2D), a chimera overlaid grid system (PEGSUS), and a
constrained numerical optimizer (DOT).
Rarefied Gas Dynamics
S. M. Yen,* K. D. Lee*
University of Illinois
A Monte Carlo simulation technique is applied to solve directly the
Boltzmann equation for rarefied gas problems encountered in modern
aerospace vehicle design. The solution procedure involves two explicit
iterative steps. First, the collision integrals are evaluated at each
computational node to define source terms in the Boltzmann equation.
The source terms are then used to integrate the time-dependent
multidimensional Boltzmann equation for the distribution function at
the next time step. Steady-state solutions are achieved as a time
asymptote. This integration scheme is well-suited to present-day
supercomputers, which employ vectorization and parallel processing.
The method is being validated for cases of simple geometry and
solutions are to be compared with those from the continuum approach,
e.g., Navier-Stokes solutions.
Aeroelastic Optimization of Smart Bleeding System for Supersonic Inlet
P. Geubelle,* E. Loth,* D. Tortorelli,* B. Wood
Computational Science and Engineering Program
The preliminary computational design of a smart bleeding system for a
supersonic inlet is performed through detailed 2-D aeroelastic
simulations using the finite-element method. Recent developments in
the use of the finite-element method in optimization problems will
also be incorporated to achieve the bleeding system design resulting
in optimal flow performance.
Smart Aeroelastic Transpiration for Shock Boundary Layer Interaction
E. Loth,* P. Geubelle, D. Tortelli, B. Wood
Computational Science and Engineering Fellowship
The study involves analysis and optimization of a novel concept to
control shock boundary layer interaction (SBLI). The system consists
of passive bleed/blowing cavity with a matrix of small flaps which
will deflect to optimize transportation, but will revert to a flat
plate in subsonic flow.
Stability and Bifurcation Behavior of Nonlinear Rotating Systems
N. Sri Namachchivaya,* R. McDonald, V. Nolan
U.S. Air Force Office of Scientific Research, AFOSR-94-1-0381
The behavior of nonlinear rotating systems under the action of an
external forcing function are investigated. The stability and
bifurcations of the periodic motions near the stable equilibrium
states are examined, in the conservative case, using the method of
averaging. All possible resonance cases will be considered. In the
nonconservative case, the method of normal forms is applied to study
the response. In addition, it is shown that the conservative system,
under additive combination resonance, undergoes a double Hopf
bifurcation, and the linear operator is nonsemisimple. The normal form
for this generalized bifurcation is derived explicitly. The global
bifurcations of general gyroscopic systems are currently being
examined using the Melnikov and extended Melnikov methods.
Delay-Differential Equations
N. Sri Namachchivaya,* H. Van Roessel* (Univ. of Alberta), Y. Liang
University of Illinois; University of Alberta; Electric Power Research
Institute, WO-8033-01
Unlike systems modeled by o.d.e.'s, mechanical systems modeled by
delay-differential equations are, in some sense, infinite-dimensional.
In this infinite-dimensional setting, the Hopf bifurcation theorem is
not an elementary exercise, but requires some new ideas and some
sophisticated mathematical machinery for its implementation. We are
studying the stability and bifurcations associated with delay-
differential equations with multiple delays. In addition, we shall
examine the stability and bifurcations associated with stochastic
time-delay systems. For a certain class of affine delay equations we
obtain Lyapunov exponents. However, for general second order systems
with delay we apply Lyapunov-like functions and the theory of
differential inequalities in order to obtain stability in the th
mean.
Stochastic Bifurcations
N. Sri Namachchivaya,* L. Arnold* (Univ. of Bremen), Y. Liang
National Science Foundation, MSS 90-57437
In this study, stochastic bifurcation implies either qualitative
changes to the invariant measures that can be observed by examining
the Fokker-Planck equation, or the appearance of a new invariant
measure which is, at present, generated numerically through the
forward and backward solutions of the stochastic differential
equations. We examine various concepts to describe stochastic
bifurcations, namely the P-bifurcation and D-bifurcation. The analysis
is carried out through studying a noisy Duffing-van der Pol oscillator
which exhibits a variety of co-dimension one bifurcations along with
certain global bifurcations in the absence of noise.
Global Behavior of Nonlinear Aeroelastic Flat Panels in Supersonic
Flow
N. Sri Namachchivaya,* C. H. Koh
U.S. Air Force Office of Scientific Research, 96-1-0265
The interaction of aerodynamic forces with a flexible structure such
as a panel can create complicated vibrational effects that may
adversely affect the overall performance of an aircraft. Large-
amplitude flutter, buckling, and fatigue failure are all possible
results of the flow-induced dynamics. We investigate the effect of
nonlinearities on the global dynamics of flat panels in supersonic
flow. The first part involves modeling the aerodynamic forces and
moments that act on the flight vehicle and the nonlinearities that are
inherent in the panel. The second part is concerned with the effects
of boundary-layer turbulence on the panel dynamics.
Nonlinear Dynamics of a Spinning Disc
N. Sri Namachchivaya,* N. Malhotra (Caltech)
U.S. Air Force Office of Scientific Research, 93-1-0063
Flexible discs rotating at very high speeds can have large transverse
deflections and thus the nonlinear effects can play a significant role
in determining the dynamics of the rotating system. The objective of
this investigation is to examine the nonlinear dynamics of a circular
spinning disc rotating at a variable spin rate. The properties of
symmetry groups are being used to examine the complex motion of this
system. These properties simplify the system considerably and reveal
vital information about the stability behavior even before extensive
analysis is performed. The local and global dynamical behavior will
also be examined numerically to confirm the theoretical predictions.
One of the main motives in this study is to analytically identify the
domains in the parameter space where the spinning disc system may
display chaotic vibrations.
Control and Dynamics of Nonlinear Systems
N. Sri Namachchivaya,* P. Voulgaris,* R. McDonald
National Science Foundation, MSS 90-57437
This project deals with the control and dynamics of nonlinear
dynamical systems. The stability properties of the uncontrolled system
are determined. Providing the system can be stabilized, nonlinear
control will be performed using recently developed geometric control
techniques. In addition to control design, the project deals with the
nonlinear analysis of the resulting feedback system. The effects of an
input noise will be considered also. Examples of possible nonlinear
control problems include the control of rotating systems, flutter, and
large deformations.
Sample Stability of Stochastic Dynamical Systems
N. Sri Namachchivaya,* L. Arnold* (Univ. of Bremen), V. Nolan
U.S. Air Force Office of Scientific Research, 93-0041; Electric Power
Research Institute, WO-8033-01
Lyapunov exponents are a generalization of the characteristic Floquet
exponents so that more general nonperiodic orbits can be
characterized. The spectrum of Lyapunov exponents provides the average
exponential rates of divergence or convergence of nearby orbits in
phase space. In addition, the Lyapunov exponents characterize the
almost-sure stability of dynamical systems perturbed by noise.
Furthermore, the moment Lyapunov exponent describes the moment
stability of such systems. Asymptotic expansions of these exponents
are constructed for stochastic dynamical systems when the noise (white
or colored) is small. These results are used to understand stability
and bifurcation characteristics of stochastic systems and are applied
to various practical dynamical systems.
Experimental Studies in Nonlinear Mechanical Systems
N. Sri Namachchivaya,* V. Nolan
National Science Foundation, MSS 92-12959; Tektronix Inc.; U.S. Air
Force Office of Scientific Research, 93-0041
This research project involves the development of a laboratory
facility to investigate the complex interactions between noise,
stability, and nonlinear dynamics inherent in mechanical systems.
Specifically, the transverse vibrations of a rotating shaft under
pulsating axial loads and the dynamics of a thin flexible disc under
time-varying rotation rates are examined near critical parameter
values. Experimental measurements will include data leading to
statistics such as mean square responses and power spectral densities;
Lyapunov exponents, fractal dimensions and correlation functions of
the response coordinates; and probability density and distribution
functions.
Stability of Tethered Spacecraft
N. Sri Namachchivaya,* R. McDonald
University of Illinois
Tethered spacecraft systems (TSS) have many applications ranging from
the generation of electric power for a spacecraft to providing
microgravity environments for experiments. The equations of motion
governing the three dimensional motion of a two-body tethered system
are highly nonlinear, yet most research on such systems has only
considered the linear equations. We propose to develop a low-
dimensional nonlinear model for such systems to analytically study the
stability, bifurcations, and chaos that may exist in these nonlinear
systems. These analytic results will then be compared to numerical
solutions to validate the low-dimensional model.
Nonlinear Dynamics of Flexible Spinning Discs
N. Sri Namachchivaya,* N. Ramakrishnan
National Science Foundation, DMS-96-10456
Under consideration are the transverse dynamics of a high-speed
spinning disc, clamped at its inner radius and rotating with a time-
varying spin rate, such as found in turbine rotor systems and computer
memory storage devices. Dynamical disturbances from time-varying spin
rates, interactions with external dynamical systems, and inherent
nonlinear effects may lead to disc instability. Such instabilities
reduce the performance of the rotating system and can, in extreme
cases, lead to failure. To examine these phenomena, the equations of
motion of the system will be derived in detail by including the
effects of geometry, material characteristics, and aerodynamic
interactions on disc displacement. Local and global bifurcation
analyses will then be implemented.
Nonlinear Dynamics of Parametrically Excited Gyroscopic Systems
N. Sri Namachchivaya,* R. McDonald
U.S. Department of Energy, NSN-2405 Antic
Gyroscopic systems occur in many areas of engineering; some examples
are pipes conveying fluid, axially loaded rotating shafts, and systems
subjected to Lorenz forces. Because of their widespread usage, their
stability characteristics are important in engineering application.
Often the required stability analysis is complicated by parametric
excitations. The effects of such excitations upon the stability of
gyroscopic systems will be investigated through application of local
and global stability analysis to discretized equations of motion
incorporating the effects of symmetry breaking and linear and
nonlinear dissipation. The results obtained will be applied to several
systems, analytically and numerically, and will be verified using a
rotating shaft experimental apparatus.
Computerized Flight Vehicle Synthesis
H. H. Hilton*
University of Illinois
An overall systems concept using an integrated approach incorporating
basic aerodynamic, guidance, control, propulsion, and structural
principles is being used to develop comprehensive generalized
simulation computer programs for flight vehicle synthesis. The purpose
is to develop educational and research tools to be used in the
teaching of and research in flight vehicle synthesis and optimization.
Current capabilities include space vehicle flight programs, airplane
missions, various structural programs to determine minimum weight and
optimum construction, and printed and terminal graphical output on
Calcomp. Interactive plotting programs for graphical display of
computational results have been developed and are operational.
Human-powered Watercraft
S. White*
University of Illinois; Society for Advanced Materials and Process
Engineering
A human-powered watercraft is being designed and built to break the
world speed record for this class of vehicle, which currently stands
at 18.5 knots (23.4 miles per hour). The vehicle is a hydrofoil, and
composite materials are being used extensively in its construction. A
systems type of engineering approach has been used to design the
vehicle drawing on principles of propulsion, aero/hydrodynamics,
structural mechanics, biomechanics, and materials. New types of
materials, manufacturing techniques, and components are being utilized
to push the speed of the vehicle above the 20-knot barrier.
Effect of Mixing on HF Overtone Laser Efficiency
L. H. Sentman,* A. Eyre, J. Cassibry, B. Wootton
W. J. Shafer Associates
To investigate the role of mixing on HF overtone laser efficiency, a
new nozzle, which injects the helium and hydrogen through orifices in
the side wall of the supersonic portion of the nozzle, was designed
and constructed. Preliminary tests indicate that the fundamental
performance of this nozzle is twice that of the parallel slit nozzle.
The estimated overtone efficiency is similar to that of the parallel
slit nozzle.
Line Selected Oscillator Performance
L. H. Sentman,* S. J. Gordon, A. J. Eyre, J. Cassibry, B. Wootton
W. J. Shafer Associates
A diffraction grating was used in the Littrow and off-Littrow
configurations to select the lines in a cw HF stable resonator. The
beam is outcoupled through a partially reflective mirror. Single-line
Littrow, single-line off-Littrow, and off-Littrow line pair
performance were measured. For the first time, as far as we know, in
the operation of chemical lasers, Littrow and off-Littrow lines lased
simultaneously. Two-line (one Littrow line, one off-Littrow line) and
three-line (one Littrow line, two off-Littrow lines) Littrow/off-
Littrow line combinations were demonstrated. The grating reflectivity
was a strong function of the polarization of the laser beam. Data
indicated that the grating generated a beam whose polarization was
perpendicular to the grating grooves.
Large-Scale CFD for Supersonic Systems
L. H. Sentman,* W. C. Solomon,* D. L. Carroll,* M. Sotano, T. Madden
Defense Advanced Research Projects Agency; Logican; Aerosoft
DARPA and the U.S. Air Force are sponsoring large-scale computational
research for parallel computation. Large-scale 3-D reacting flow
models are developed to support advanced laser research at Phillips
Lab and air-breathing propulsion research underway at the Air Force
Propulsion Lab.
High-Energy Laser Systems Technology
L. H. Sentman,* W. C. Solomon, D. Carroll, D. King
U.S. Air Force Phillips Laboratory; STI Optronics; TRW, Inc.
This work involves research into the establishment and operation of a
large-scale COIL laser for commercial applications. A new laboratory
is being constructed for establishing a commercial technology for
high-energy lasers. The initial project is the development of a
prototype system to be used in laser processing of metals. This
problem involves the development of a new laser technology,
demonstration at a scaleable power level, and materials processing of
lasers operating near 1 micrometer wavelength. Initial operation of
the system demonstrated a kilowatt at 25% chemical efficiency.
Large-Scale Simulations of Chemical Lasers
W. C. Solomon,* T. Madden, M. Gunawan, D. Stromberg, L. Fockler
Aerosoft Corp.; Defense Advanced Research Projects Agency
Large 3-D simulations of chemical laser flowfields are being conducted
to determine the most efficient configurations of hardware and
software for the next generation of computing. Verification of
experimental data and earlier, less complex computations is complete,
and the system performance is being assessed. The work is being
conducted using the reacting flow model provided by the GASP CFD
simulation on a variety of parallel and high-speed processors.
An Experimental Investigation of the COIL Laser
W. C. Solomon,* L. H. Sentman,* D. L. Carroll,* D. King
STI Optronics; U.S. Air Force Research Laboratory, AF 95111
A subscale model of an advanced chemical laser is operating to provide
information on the proper design of a full-scale device. Designs taken
from the DARPA effort are being incorporated into the experiment. A
series of experiments focusing on potential DOD applications as well
as on a database for the commercial development of the technology are
underway. This is providing basic understanding of the technology, a
useful database for the CFD modeler, and a highly flexible testbed to
pursue new approaches to find more efficiency as well as better
quality beams and delivery systems.
Quenching Effects in Continuous Curing of Thermoset Composites
J. Buckmaster,* S. White*
U.S. Air Force Office of Scientific Research, F49620-96-1-0031
Continuous curing refers to a process in which lay up occurs
contemperaneously with cure, with a well-defined cure front
propagating through the freshly laid up material. The initiation,
propagation, and stability of the cure front all need to be understood
for the successful development of the process. This theoretical
project is concerned with the initiation process where numerical
simulations have suggested that "ignition failure" can occur
in a cylindrical geometry if heat losses from the lay-up surface are
too large.
Self-Heating Effects in Thermoset Composites
J. Buckmaster,* T. G. Vedarajan
U.S. Air Force Office of Scientific Research, F49620-96-1-0031
The curing of thermoset composites requires an elevated temperature,
but one not too elevated. Because the process is exothermic, self-
heating occurs, and, for thick specimens, self-heating can lead to
temperatures that are too high. A criterion has been developed, based
on Frank-Kamenetskii ignition theory, that defines "thick"
in this context. Also, flame-like models have been developed that
permit the study of cure front propagation and stability in a simple
fashion for arbitrary cure kinetics.
Impact-induced Damage of Composite Structures
P. Geubelle,* J. Baylor
University of Illinois
The impact resistance of polymeric matrix composites has always been a
major factor limiting their wider use in structural applications.
Unlike metallic structures which experience localized surface
(indentation) damage, composites dissipate the impact energy by
creating large internal surfaces (delamination), which strongly affect
their post-impact structural strength. The objective of this research
is to develop a special finite-element scheme to accurately model the
delamination behavior and predict the location and extent of the
damage.
Spectral Method for 3-D Dynamic Interfacial Fracture
P. Geubelle,* M. Breitenfeld
University of Illinois
The numerical simulation of 3-D dynamic fracture events is one of the
most challenging computational issues in solid mechanics because of
the extreme refinement needed to capture continuously evolving
geometries (as the fracture surface extends) and rapidly moving stress
waves. A spectral method, recently introduced to study the spontaneous
propagation of planar crack and faults of arbitrary shapes and
subjected to arbitrary loading conditions, provides a very efficient
numerical tool. In this research project, extension of the spectral
scheme to dynamic failure of interfaces is examined. Various issues
such as the existence of limiting propagating crack speeds along weak
interfaces will be investigated.
Failure of Layered Brittle Systems
P. Geubelle*
University of Illinois
Layered systems are being considered more frequently in a wide range
of applications involving time-dependent loading conditions. They are
used, for example, in the protection of structures subjected to impact
loading conditions for their ability to absorb an important part of
the kinetic energy of the impactor, thereby reducing the damage of the
substrate. Layered systems have also been proposed as passive damping
systems of various structures subjected to harmonic loading. This
research project is aimed at the investigation of the dynamic failure
of this type of structure, using a special cohesive-based finite-
element scheme.
Role of the Cohesive Failure Model in Quasi-Static and Dynamic
Fracture
P. Geubelle*
University of Illinois
Cohesive failure models are often used in the analytical and numerical
study of spontaneous crack propagation. These simple models have been
shown to capture some important dynamic fracture effects, such as
maximum crack speed, crack branching, and unsteady crack energetics.
The primary objective of this research project is to gain a better
understanding of the importance of the often ignored rate-dependent
effect on the spontaneous propagation behavior of a crack. The
numerical tool used in this study is a spectral form of the boundary
integral formulation of the elastodynamic relations which allows for
the incorporation of a wide range of cohesive models.
Transgranular Dynamic Failure of Solid Propellants
P. Geubelle,* R. Haber*
DOE Center for the Simulation of Advanced Rockets
The issue of dynamic failure of a particulate composite such as a
solid propellant is the topic of this research project, which aims at
the development and parallel implementation of a special form of the
finite-element scheme able to simulate the spontaneous transgranular
crack propagation. Of particular interest are the effect of the
microstructure on the crack propagation and the choice of the rate-
dependent cohesive failure model used to simulate the failure of the
polymeric binder. The problem is complicated by the continuous burning
and pressurization of the crack faces by the reacting gases.
Dynamic Fiber Pull-Out in Polymeric Composites
P. Geubelle,* X. Bi, J. Lambros (Univ. of Delaware)
National Science Foundation, CMS-9712291
When a composite structure fails and a crack propagates
perpendicularly to the fiber direction a substantial amount of energy
is dissipated in the progressive debonding and sliding on the fibers.
Preliminary experimental investigations have shown that these two
processes can be quite different under high strain rate conditions
than in a quasi-static situation. In this combined experimental and
analytical research program, the dynamic failure of a model composite
is examined using a specially adapted version of the Split Hopkinson
Bar apparatus and a special form of the finite-element scheme.
Probabilistic Minimum Weight Analysis
H. H. Hilton*
University of Illinois
An analytical method has been developed for designing structures
having a prescribed probability of failure so that the overall weight
is minimum under combined loads. The solution is obtained for
structures consisting of components having normal, Weibull beta-
distributed applied and failure stresses, and is applicable to
combined loading conditions. The loading conditions are such that
general relations can be used to relate the mean stresses to the
cross-sectional area. Weight comparisons with standard design
procedures based on the margin of safety concept are made and indicate
the possibility of substantial weight savings.
Random Viscoelastic Material Effects
H. H. Hilton*
University of Illinois
Analytical studies are presented which extend the elastic-viscoelastic
analogies to stochastic processes caused by random linear viscoelastic
material properties. Separation of variable as well as integral
transform correspondence principles is formulated and discussed in
detail. The statistical differential equation of the moment
characteristic functional is derived, but rather than solving the
highly complex functional equation, the solutions are formulated in
terms of the first- and second-order statistical properties. Gaussian,
Weibull, and beta distributions are considered for the probability
density distributions of creep and relaxation functions; and their
effectiveness is evaluated.
Analytical Determination of Optimum Viscoelastic Material Properties
H. H. Hilton,* S. Yi*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
The influence of complex modulus shapes and parameters on creep,
relaxation, and damping is being investigated. These moduli will be
used to solve dynamic and static problems such as bending, torsion,
and flutter of lifting surfaces. The results will yield a
categorization of viscoelastic material behavior in its relation to
creep, relaxation, and damping. Both isotropic and anisotropic
materials are being considered. Such an analytical catalog of material
behavior then can be employed to fabricate real materials to conform
to such modulus specification. Selections of these materials has
direct application in the design of sound proofing, shock absorbers,
composites, and helicopter blades.
Mechanics of Processing--Fabrication of Thick Composite Materials
S. White,* R. Lindberg
University of Illinois
Thick polymer composites are difficult to process. They are prone to
thermal spiking, excessive processing time, nonuniform curing and
consolidation, and high residual stresses. In this research, new
manufacturing techniques are being developed to solve these problems.
The underlying processing science is being investigated using a
combined analytical and experimental approach. Processing models are
formulated to explain and predict the behavior of thermosetting
composites during cure, and experiments are conducted to validate
these models. The research is multidisciplinary, drawing from
principles in solid and fluid mechanics, thermodynamics, polymer
chemistry, and heat transfer.
Design and Manufacture of Adaptive Structures
S. White,* J. Berman
University of Illinois
Adaptive structures and materials sense their environment and react to
these sensory inputs in some logical fashion. The sensor/actuator
materials for these applications can take many forms: shape-memory
alloy wires, piezoelectric patches, fiber optics, etc. This research
investigates the design and manufacture of shape-memory alloy
composites, piezoelectric composites, and hybrids combining the two
sensor/actuators. The approach builds upon investigations at the
microlevel (interfacial bonding, residual stresses) to the macrolevel
(structural mechanics of beams/tubes, process modeling). These types
of materials have wide-ranging applications in the civil
infrastructure, aerospace, and automotive industries.
Use of Corn Byproducts for Structural Composite Materials
S. White,* N. Sottos (Theoret. & Appl Mech.), T. Mackin (Mech.
& Indust. Engr.)
State of Illinois
Two important issues have gained national priority in recent years:
the development of alternative markets for corn and its byproducts and
the revitalization of the U.S. civil infrastructure. These two issues
are synthesized in the current project that focuses on the development
of cheaper composite materials for civil engineering applications by
using corn byproducts as reinforcements in polymer matrix composites.
Husks, fiber stalk, kernels and fiber silks are mechanically tested
both individually and as embedded reinforcements in several different
polymer matrices. Once a reasonable database has been established for
the mechanical properties of corn byproducts, their potential for
structural composites will be evaluated.
Smart Tagged Composite Materials
S. White,* F. Browers, J. Hommema
U.S. Army Construction Engineering Research Laboratories, DACA88-97-K-
0001
Knowledge of the health of a material or structure is critical for
timely maintenance and repair of components. This issue is especially
difficult for composites because subsurface flaws are hidden from
visual inspection. By incorporating smart material tags into the
matrix of a polymer composite and then interrogating these tags, the
state of health of a composite structure can be qualified throughout
its processing and service life. Tagging materials investigated
include piezoelectrics, shape-memory alloys, electrostrictives,
ferromagnetics, and magnetostrictive materials.
Self-repairing Polymeric Composites
S. White,* N. Sottos* (Theoret. & Appl. Mech.), P. Geubelle,* M.
Kessler, J. Hommema
U.S. Army Corp of Engineers Construction Engineering Research
Laboratories, DACA88-95-D-0004-2
When a plant or animal is injured, its systems secrete certain
substances to cause filling, healing, or regeneration at the site of
the injury. Can structural composite materials be designed to mimic
this type of healing behavior? Our research is focused on the
development of self-healing concepts for polymeric composite
materials. One such concept is the incorporation of hollow polymer
microspheres filled with a repair agent. Once a crack occurs in the
material, the repair agent would release and fill the cracks,
rebonding the crack surfaces together.
Measurement of Dynamic Crack Growth Rates with Composite Tagging
Techniques
P. Geubelle,* S. White,* F. Browers
University of Illinois
The measurement of the growth rate of cracks propagating dynamically
in the interior of a composite structure is a very difficult task. In
fact, no such experimental technique has been developed yet. We are
attempting to incorporate small magnetostrictive particles in a
polymeric material in order to measure the stress wave as it passes
through the material while a crack propagates. The stress wave imparts
a local rise in the magnetic field surrounding the magnetostrictive
particles and this change in the magnetic field is sensed. Crack
growth rates can be calculated by using multiple magnetic field probes
on the material surface.
Dimensional Stability and Process Cycle Optimization of Composites
S. White,* P. Geubelle,* C. Tucker III* (Mech. & Indus. Engr.), D.
O'Brien
National Science Foundation, DMI 96-10382
Residual stresses developed during the manufacture of composites have
a strong influence on the final shape of the manufactured part. A
precise understanding of the phenomena leading to the appearance of
these residual stresses is the primary objective of this project, in
which special emphasis is placed on characterizing the polymeric
matrix composite during the manufacturing process. A finite-element-
based numerical tool will be combined with recent developments in
sensitivity and optimization analysis. The objective is to optimize
the processing conditions and shape of the manufacturing tool in order
to achieve the desired final shape of the composite part.
Nonmechanical Deformation of Curing Polymers
S. White,* C. Fischer
University of Illinois
Thermosetting polymers during cure shrink by as much as 10% as a
result of the irreversible chemical reactions that take place. Such
large deformations can lead to high residual stresses in these
materials. An experimental study is underway to measure the
development of these deformations during cure. Of particular interest
is resolving the individual contributions of both thermal and chemical
processes to the volumetric deformation.
Thermomechanical Design of SMA Composites
S. White,* A. Al-Aql*
University of Illinois
The use of shape-memory alloys (SMA) in composites gives the designer
more control over tailoring of thermomechanical properties. In this
research SMA composites are being designed so that the thermal
expansion behavior of the material can be tailored over specific
temperature ranges. By using different types of SMA materials, a
composite can be designed that exhibits both high expansion and low
expansion behavior triggered at predefined onset temperatures.
Stability of Flows through Porous Media at Large Reynolds Number
R. A. Beddini,* C. Low
University of Illinois
Porous walls are often used for the surfaces of rocket chambers and
their experimental simulators. It has been experimentally observed
that as the Reynolds number of the flow based on pore size becomes of
the order 10 or more, the flow becomes unstable, with large magnitude
nonuniformities. These disturbances are observed to be nearly random
spatially, but temporally steady--a "pseudo-turbulence"
phenomenon that has not been analytically addressed in the literature.
The mechanism is presently believed to be related to the convective
drag terms in the equations of motion. Linear instability techniques
are being employed to analyze the flow.
Analysis of Mean Flow and Turbulence Effects on Acoustic Response
Threshold in Solid Propellant Rockets
R. A. Beddini,* Y. Lee
California Institute of Technology, CIT subcontract
Nonlinear stability theories require and depend strongly on what is
traditionally termed a velocity response function. Through
computational solutions of a turbulence and combustion model, prior
work has shown that a significant source of response to solid rocket
aeroacoustics can result from the interaction between acoustically
induced turbulence and combustion processes. This research effort will
undertake an approximate analysis of the effects of mean flow on the
velocity response threshold condition.
Solid Propellant Flame Geometry
J. Buckmaster,* J. Yao
U.S. Air Force Office of Scientific Research; F49620-96-1-0031;
Ballistic Missile Defense Organization/U.S. Office of Naval Research,
N00014-95-1-1339
Models have been developed and are being used to elucidate the
geometry of the flames that occur in the burning of heterogeneous
solid propellants. Of particular importance are flame heights and the
location of each flame base relative to the fuel/oxidizer interface at
the propellant surface. Also of importance is the structure of each
flame base, for this plays a crucial role in defining the heat flux to
the propellant which sustains the burning.
Modeling of Composite Propellant Burning and the Response to Pressure
Disturbances
J. Buckmaster; M. Q. Brewster and H. Krier (Mech. & Indus. Engr.)
U.S. Air Force Office of Scientific Research, F49620-96-1-0031; U.S.
Office of Navy Research, N00014-95-1-1339
New models of composite propellant burning are being developed,
exploiting recent investigations of edge flames, structures that have
some of the characteristics of premixed flames and some of the
characteristics of diffusion flames. The first goal is to learn about
the dependence of the burning rate on pressure and on the scale of the
propellant morphology. A subsequent goal is to determine the response
of the burning rate to unsteady pressure disturbances.
Combustion Near the Surface of Solid Propellants
J. Buckmaster, D. S. Stewart* (Theoret. and Appl. Mech.)
DOE Center for Simulation of Advanced Rockets
The center is concerned with the simulation of an entire solid
propellant rocket system, the combustion, the gas flow and acoustics,
the structure, and the material behavior. Different groups in various
departments of the college are responsible for different ingredients
in this study. We are concerned with the combustion processes that
occur near the surface of the solid propellant and their detailed
numerical simulation.
Shock Ignition of Metals in Solid Propellant Combustion Products
R. L. Burton,* H. Krier,* S. Kochevets, M. Spalding
U.S. Office of Naval Research, Ballistic Missile Defense Organization,
N00014-95-1-1339 (In conjunction with the Department of Mechanical
and Industrial Engineering)
New energetic solid propellants will contain metals such as aluminum,
magnesium, and boron. Experiments in a high-pressure shock tube
measuring boron ignition delay and combustion (burn) time, as well as
measurements by emission spectroscopy of the transient reactive
species, in the range of 0.5 to 5 MPa and temperatures ranging from
1800 K to 3000 K, will test various theories of chemical processes for
boron fuel utilization in propellant combustion gases. Also,
experiments in a high-pressure windowed flow reactor measure aluminum
particle combustion rates in propellant gases. The work will primarily
impact chemical kinetic theories of reaction pathways for such two-
phase mixtures.
Low-Power Propulsion for Small Satellites
R. L. Burton,* M. J. Wilson, S. Bushman
U.S. Naval Research Laboratory, N00014-95-1-G041; Unison Industries;
TRW, Inc.; Industrial Affiliates Program
Low-power propulsion has been identified for orbit-raising
applications on small satellites. A pulsed Teflon plasma thruster is
being studied, both experimentally and theoretically, for the 10-100
watt power range, which would be capable of both maneuvering and orbit
raising missions. The effort is primarily focused on increasing the
notoriously low efficiency of this type of thruster.
Plasma Processes in Solid-Fed Pulsed Plasma Microthrusters
R. L. Burton,* S. Bushman, E. Antonsen
U.S. Air Force Office of Scientific Research, F49620-97-1-0443,
F49620-97-1-0138
Advances have been made in the performance of the pulsed plasma
thruster (PPT) with new approaches to the external circuit and to the
thruster design, enabling significant increases in the thruster
impulse and efficiency at reduced pulse energies. Experiments are
performed to measure the current, density, and temperature
distribution within the thruster nozzle, as well as density outside
the thruster, in order to understand the plasma acceleration process.
A parallel plasma modeling effort is being performed to make further
improvements in thruster performance.
Multipass Herriot Cell for Density Diagnostics in Pulsed Thrusters
R. L. Burton,* S. Bushman, E. Antonsen
U.S. Air Force Research Laboratory
The low neutral density in the exhaust of a pulsed plasma thruster
(PPT) can be measured if the optical path length is sufficiently long.
This measurement, requiring at least 20 passes of a laser beam through
the exhaust plume of the PPT, can be achieved with a Herriot cell,
capable of up to 100 passes. This research effort will design and
fabricate a Herriot cell for PPT density measurements and will perform
the measurements under typical operating conditions. The resulting
measurements will be used to validate gas dynamic models of PPT
operation.
Turboramjet Propulsion
W. C. Solomon,* J. Keen, M. Gunawan, D. Carroll
U.S. Air Force Aeropropulsion Laboratory, F33615-89C-2912
Research is being conducted to develop an understanding of
nonequilibrium phenomena associated with fuels in high-speed ramjets.
Modeling of combustion experiments being conducted by the air force is
providing new insights into the performance of high-speed ramjets. A
nonequilibrium model is being compared to data.
Nonequilibrium Models for Air-breathing Turboramjets
W. C. Solomon,* M. Gunawan, D. Lewis
National Aeronautics and Space Administration, NGT-40017
Synthetic mixtures of hydrocarbons are being combusted in air to
develop full reaction kinetic models. These models are being
simplified for insertion into full CFD descriptions of internal flow
systems.
Performance of Laboratory-Scale Hybrid Rocket Performance
W. C. Solomon,* J. Norr
National Aeronautics and Space Administration, NGT-40017
A hybrid rocket designed to produce 300 lb of thrust is being prepared
for test firing. This rocket is a moderately instrumented subscale
derivative of higher thrust designs. The investigation of the
performance includes thrust measurements and overall impulse
measurements. The effects of grain design, formulation, and ignition
properties on combustion and stability performance are being
investigated. Improvements in performance and operational parameters
will be explored.
An Assessment of the International Space Station's Dependence on Soyuz
and Progress Spacecraft Operations
V. Coverstone-Carroll,* R. Davis
University of Illinois; NASA Johnson Spaceflight Center
This study provides a general overview of the flight profile and key
subsystems for the Russian Soyuz and Progress spacecraft, with an
emphasis on the operationally significant limitations of each craft.
Key subsystems investigated include GN&C, propulsion, life
support, onboard computers, and thermal control. The dependence of the
International Space Station program on Soyuz and Progress operations
is addressed. The study focuses on the following key risks to cost and
schedule: inherent design limitations, general space infrastructure
decline (manufacturing, launch operations, flight control, training),
and the general instability of Russian politics and financing. Source
materials include original Russian technical and training materials
from the Russian space corporation, Energia, and the Gagarin Cosmonaut
Training Center.
Stochastic Differential Equations
L. A. Bergman,* S. F. Wojtkiewicz, B. F. Spencer, Jr.* (Univ. of Notre
Dame)
National Science Foundation, ECS 92-24828
The Fokker-Planck equation, a convective-diffusion equation governing
the evolution of the transition probability density function of a
class of dynamical systems subjected to both additive and
multiplicative excitations exhibiting Markovian response, has been
solved successfully by a finite-element method for certain second-,
third-, and fourth-order systems. Both domain decomposition and
iterative solvers with preconditioning have been applied to the
problem. The development of post-processing software to compute
marginal density functions, n-th order moments of response, and
extremal statistics associated with these distributions is now
complete. Visualization methods have been used to examine the details
of resulting probability flows in the phase space. Current research is
directed at higher dimensional systems.
Methods for "On-the-Fly" System Identification
L. A. Bergman,* P. G. Voulgaris,* E. A. Johnson
NASA Dryden Flight Research Facility, NAG 2-4001
It sometimes may be desirable to monitor the response of one or more
modes of a system to warn of the onset of pathological behavior
associated with a single mode, such as flutter. This is not
straightforward, particularly if the goal is to perform on-line, or
"on-the-fly," monitoring in the presence of unmodeled
dynamics, unknown external forces, rapidly changing control forces,
and various noise sources associated with real measurements. A two-
stage algorithm has been developed which takes into account the slowly
time-varying nature of the system. The outer loop employs an ARMAX
procedure, while the inner loop is essentially a modal Kalman filter.
Convergence of the algorithm has been demonstrated in simulation under
a variety of conditions.
Covariance Control
L. A. Bergman,* R. V. Field, Jr.
University of Illinois; Sandia National Laboratories
A method has been developed for covariance control of large-scale
linear structures under reliability constraints. The work builds on
the method of Skelton for covariance control, but employs procedures
based on Rice's method for the companion reliability problem given by
vector outcrossings from a polygonal safe region to define target
covariances.
Torsion-bending Flutter of Viscoelastic Wings
H. H. Hilton*
University of Illinois
An analysis of subsonic and supersonic torsion-bending flutter,
including rotary inertia, shear, and hearing effects, of a time-
dependent linear viscoelastic lifting surface consisting of either a
Bernoulli-Euler or a Timoshenko beam is formulated using aerodynamic
strip theory. Complex moduli models for aluminum are characterized as
functions of temperature and frequency by fitting Chebyshev
polynomials to actual material experimental data. The flutter analysis
is carried out in the complex plane and a computerized iterative
method for the determination of flutter speeds and frequencies is
developed. The influence of viscoelastic material properties (storage
and loss moduli), temperature, rotary inertia, and shear effects is
evaluated.
Generalized Viscoelastic 1-DOF Deterministic and Stochastic Nonlinear
Oscillators
H. H. Hilton,* S. Yi*
University of Illinois; Nanyang Technological University
In this study, the theory of deterministic and stochastic generalized
viscoelastic Duffing, Roberts, and van der Pol oscillator responses
are formulated and evaluated. Numerical solution protocols are
developed and the results are evaluated to determine the influence of
viscoelastic damping on the oscillators' performance. It has been
found that generalized viscoelastic material behavior profoundly
affects the displacements and phase relations of these oscillators.
Acoustic and Viscoelastic Wave Propagations with Absorbing Boundaries
H. H. Hilton,* M. J. Yedlin*
University of Illinois; University of British Columbia
The previous work of Yedlin and Luo is extended and generalized to 1-D
and 2-D linear viscoelastic wave propagations with absorbing
boundaries. Formal analytical solutions are developed, showing that
the governing relations and BCs for the 1-D and 2-D problems are of
sufficient complexity that they are not amenable to analytic
solutions. However, numerical formulations using finite elements and
finite differences are employed yielding excellent results.
Appropriate solution methodologies are discussed and evaluated, and
the influence of the various viscoelastic material parameters is
examined in detail by illustrative examples. It is also shown that, as
inverse problems, these formulations and their attendant solutions can
be readily used for experimental material characterizations.
Finite-Element Analysis of Anisotropic Viscoelastic Composites
H. H. Hilton,* S. Yi,* M. F. Ahmad*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
Advanced composite laminates are being used in flight vehicles to
improve performance by substantial structural weight savings. Present
numerical analysis requires computers with large storage and takes
long real time to complete the calculations. The method under
development uses Laplace transforms and thereby requires computer real
time use comparable to elastic anisotropic analyses. Results of
various loading conditions compare extremely well with exact
analytical solutions. Finite-element analyses for dynamic loadings on
anisotropic viscoelastic composites that save extensive computer time
and storage have been developed. The numerical results compare
extremely well with analytical exact solutions.
Control of Piezoviscoelastic Lifting Surfaces
H. H. Hilton,* S. Yi*
University of Illinois; Nanyang Technological University
A systematic analytical study has been initiated to investigate static
and dynamic control of lifting surfaces through material and electric
damping and control. Sensitivity studies to determine significant
parameters are in progress.
Stochastic Viscoelastic Delamination Onset Failure Analysis of
Composites
H. H. Hilton,* S. Yi*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
The analysis includes stochastic processes due to combined random
loads and random delamination failure stresses as well as random
anisotropic viscoelastic material properties including the influence
of stochastic temperature fields, moisture contents, and boundary
conditions. It is shown that times for delamination onset occurrences
in composites can be predicted probabilistically depending on any one
or all of the above conditions. For cases where deterministic criteria
predict no delamination failures, the present stochastic failure
theory indicates high probabili-ties of failure at either early or
long times depending onthe load-time relations. The effects of fiber
orientationand of number of plies on delamination probabilities are
examined.
Mathematical and Numerical Analysis Issues in Nonlinear Anisotropic
Viscoelastic Composites
H. H. Hilton,* S. Yi*
University of Illinois; Nanyang Technological University
Complete 3-D anisotropic, nonhomogeneous, large deformation,
nonlinear, viscoelastic constitutive relations are formulated
including aging, moisture, temperature, degree of cure, and change of
state effects. Anisotropic nonlinear heat and temperature relations
for cure processes are also studied. The coupled system is solved
using combined spatial finite-element and temporal finite-difference
and/or fourth-order Runge-Kutta approaches. Stochastic failure
criteria are used to determine probabilistic survival times to
delamination onset during service and manufacturing conditions. Mesh
and incremental time step sizing influences on convergence of
numerical results are evaluated, and comparisons of stresses and
deformations with experimental data are carried out.
Rupture Mechanics in Viscoelastic Media
H. H. Hilton,* P. H. Guebelle,* M. J. Danyluk
University of Illinois
A spectral scheme for viscoelastodynamic fracture problems in
antiplane shear (mode III) is studied. The numerical method is based
on a spectral representation of the exact viscoelastodynamic relations
between tractions and displacement discontinuities along the fracture
plane. It allows for a precise and efficient investigation of
dynamically loaded stationary or propagating cracks or faults in a
viscoelastic medium and incorporates a wide range of cohesive models
used to characterize the fracture process in the vicinity of the
propagating crack tip. Various viscoelastodynamic fracture problems
involving stationary and moving cracks are investigated.
Finite-Element Analysis of Coarse-grained Vector Machines of Residual
Stresses in Viscoelastic IC Packages during Surface-mounting Processes
S. Yi,* H. H. Hilton*
Nanyang Technological University; University of Illinois
Moisture and temperature distributions and residual stresses in
plastic-encapsulated IC packages are evaluated to assess product
reliability. Finite-element analyses (FEA) are done on the NTU CRAY
T94 to calculate hygrothermally induced anisotropic viscoelastic
deformations and stresses in plastic IC packages during surface-
mounting processes. Numerical results show that substantially high
stresses in silicon chips and lead frames occur when LOC TSOP packages
are exposed to reflow soldering processes. Numerical results also
demonstrate that residual stress values in IC packages are sensitive
not only to the magnitude of the loads but also to the loading history
because of the hygro-thermo-viscoelastic behavior of plastic mold
compound IC materials.
Anisotropic Piezo-Electro-Thermo-Viscoelasticity Theory with
Applications to Composites
H. H. Hilton,* J. R. Vinson,* S. Yi*
University of Illinois; University of Delaware; Nanyang Technological
University
The general, nonlinear, 3-D, large deformation theory of anisotropic
piezo-electro-thermoviscoelasticity is formulated and represents the
confluence of anisotropic elasticity and thermoviscoelasticity,
nonhomogeneous layered media and piezoelectricity. For linear
materials and small deformations, a piezoelastic/piezoviscoelastic
analogy is established in terms of integral Fourier and Laplace
transforms. To demonstrate the effectiveness of the piezoviscoelastic
constitutive relation derivations, several piezoelastic examples of
beam and plate solutions have been reformulated in terms of
piezoviscoelastic constitutive relations and solved analytically and
numerically using viscoelastic finite-element analyses. Comparisons
with piezoelastic solutions are made and sensitivity analyses of
piezoviscoelastic parameters are undertaken.
Free Edge Stresses in Elastic and Viscoelastic Composite Laminates
under Uniaxial Extension, Bending, and Twisting Loadings
S. Yi,* H. H. Hilton*
University of Illinois; Nanyang Technological University
Interlaminar stresses near free laminate edges may result in
delamination onset and growth and result from mismatches in layer
properties. Little is known about interlaminar stresses caused by
bending and/or twisting loads. A finite-element procedure for the
analysis of time-dependent interlaminar stresses in elastic and
viscoelastic laminated composites subjected to arbitrary combinations
of axial extension, bending and/or twisting loads is developed based
on displacement fields for laminates under a generalized plane
deformation state. Parametric studies are presented to demonstrate the
accuracy of the numerical procedures.
Anisotropic Viscoelastic Fractional Derivative Material
Characterization
H. H. Hilton*
University of Illinois; National Center for Supercomputing
Applications
Isotropic linear and nonlinear fractional derivative constitutive
relations are formulated and examined in terms of generalized Kelvin
models and are analytically extended to cover general anisotropic
viscoelastic behavior. Integral constitutive relations, which are
computationally more powerful, are derived from fractional
differential ones and the associated anisotropic temperature-moisture-
degree-of-cure shift functions and reduced times are established.
Approximate Fourier transform inversions for fractional derivative
relations are formulated and their accuracy is evaluated. The efficacy
of integer and fractional derivative constitutive relations is
compared and it is found that use of the former is preferable in
analyzing isotropic and anisotropic real materials.
Shear Center and Neutral Axis Motion in Anisotropic Viscoelastic
Bending
H. H. Hilton,* S. Yi,* J. R. Phillips
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
An analytical study of anisotropic viscoelastic beams (composites) is
formulated for the neutral axis location which is shown to move with
time, thus changing with time sectional areas where tensile and
compressive bending stresses occur. Such changes in location of
positive and negative bending stresses seriously affect failure
conditions such as delaminations and the onset of creep buckling
because of increased stress magnitudes in either direction. The time
motion phenomenon of the shear center and neutral axis locations is
unique to nonhomogeneous isotropic and anisotropic viscoelastic beams
and has no counterpart in elastic beam theory. The effects of material
parameter variations, fiber orientation and stacking sequences, are
evaluated in detail.
Finite-Element Analysis of Thick Thermosetting Matrix Composite
Manufacturing Cure Process
S. Yi,* H. H. Hilton,* M. F. Ahmad*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
A transient heat transfer finite-element model is introduced to
simulate the curing process of polymer matrix composites, and a three-
dimensional anisotropic cure simulation for a thick laminated
composite is performed. The temperatures inside of the laminates can
be evaluated by solving the 3-D nonlinear anisotropic heat conduction
equations including the internal heat produced by chemical reactions.
The internal heat generation term can be expressed in terms of the
cure rate. Correlation between experimentally measured and predicted
temperature gradients is presented for various cure cycle histories.
Probabilities of delamination during cure are evaluated.
Process-induced Residual Thermal Stresses and Deformations in Thick
Thermosetting Matrix Composite Laminates
S. Yi,* H. H. Hilton,* M. F. Ahmad*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
A transient heat transfer, finite-element model is introduced to
simulate the curing process of polymer matrix composites. Temperature
distributions inside the laminates are evaluated by solving the
nonlinear anisotropic heat conduction equations, which include the
internal heat produced by chemical reactions. The internal heat
generation contribution is expressed in terms of cure rates.
Correlations between experimentally measured and predicted temperature
gradients are found for various cure cycle histories. The effects of
temperature and cure rate on viscoelastic responses of graphite-epoxy
laminated composites are investigated using finite-element analyses.
Residual stresses for these composite plates subjected to temperature
cycles are also determined.
Stochastic Delamination Buckling of Viscoelastic Columns
H. H. Hilton,* S. Yi,* M. J. Danyluk
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
The effects of random failure criteria on delamination buckling are
studied under deterministic loads, geometries, moduli, temperatures,
and moisture contents. This allows for an analysis that focuses on and
isolates random delamination buckling criteria effects under otherwise
deterministic conditions. Viscoelastic failure stresses and moduli
decrease in time, while bending stresses, strains, and deformations
increase with time. Using the experimentally determined delamination
probability distributions reported by Hiel et al. in conjunction with
the combined load stochastic failure criterion of Hilton and
Ariaratnam, probabilities of delamination onset occurrences as time
functions are formulated.
Large Deflections of Linearly Elastic and Viscoelastic Columns with
Follower Loads
H. H. Hilton,* S. Yi,* T. Ruijun
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
A large-deflection analysis of nonhomogeneous linearly elastic or
viscoelastic columns with initial curvature and with variable areas
subjected to follower loads is formulated. Column end shortening due
to both curvature and compressible loads is taken into account, and
the governing coupled nonlinear differential equations are solved
numerically. The linear elastic-viscoelastic integral transform
analogy is analytically extended to this nonlinear problem. The
effects of end shortening, follower load angle, nonhomogeneous
material properties, and variable area on elastic and viscoelastic
columns are studied in detail.
Optimum Material Property Formulation for Anisotropic Viscoelastic
Damping
H. H. Hilton,* S. Yi*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
Anisotropic viscoelastic material properties are formulated
analytically taking into account fiber orientations and stacking
sequences for laminated composites. The detailed influences of
resulting anisotropic moduli are investigated in terms of material
response to loads and deformations and the ability to dissipate
energy, i.e., damp out undesirable motion.
Viscoelastic Stress Analysis of Composite Laminated Shells with
Cutouts
S. Yi,* H. H. Hilton,* M. F. Ahmad,* G. D. Pollock
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
Yi investigated the onset of delaminations in viscoelastic laminated
composites based upon the average interlaminar stress fields and the
quadratic delamination criterion. Hilton and Yi analyzed quasi-static
responses of an anisotropic viscoelastic solid under time-dependent
mechanical and hygrothermal loads. In the present study, time-
dependent responses in viscoelastic composite shell structures with
holes are evaluated based on Ahmad's degenerated shell elements.
Graphite/epoxy composite laminates are considered, and the effects of
stacking sequences and temperatures are studied. The stress and strain
fields around cutouts in viscoelastic composite shells are calculated
as functions of load and environment history.
Performance Evaluations of Viscoelastic Finite-Element Analyses on
Coarse Grained and Massively Parallel Supercomputers
S. Yi,* H. H. Hilton,* M. F. Ahmad*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
Performance evaluations of dynamic viscoelastic finite-element
procedures and codes on coarse grained and massively parallel
supercomputers such as the CRAY Y-MP, CRAY C-90, and the Connection
Machine 5 (CM-5) have been undertaken. The element stiffness
computations are in the range of 73 to 183 mega floating-point
operations per second (Mflops) on the NCSA CRAY Y-MP. Using 4 node
plane elements and the 256-processor CM-5, a speedup factor of about
22 over the CRAY Y-MP was obtained for calculating 1.6x105 element
stiffness matrices, which is 1.65 Gflops. The performances of the
conjugate gradient method on the CRAY Y-MP and CM-5 are also evaluated
and are compared with the ones of the Cray sparse matrix and of the
Feable solvers.
Thermoviscoelastic Analysis of Fiber-Matrix Interphase
S. Yi,* H. H. Hilton,* M. F. Ahmad,* G. D. Pollock
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
In this study, both elastic and viscoelastic constitutive relations
are considered for the matrix. Time-dependent strains and stresses of
the fiber, interphase, and matrix are computed and the interaction
between the fiber and matrix is examined. Effective Young's moduli for
composites are evaluated. Effects of the viscoelastic interfacial
layer on the average transverse Young's modulus are also investigated.
The numerical evaluation shows that the effective transverse Young's
modulus of fiber-reinforced composites is significantly affected by
the fiber volume faction ratio and viscoelastic behavior of the
matrix.
Nonlinear Thermoviscoelastic Analysis of Interlaminar Stresses in
Laminated Composites
S. Yi,* H. H. Hilton,* M. F. Ahmad*
University of Illinois; National Center for Supercomputing
Applications; Nanyang Technological University
A finite-element formulation for analyzing interlaminar stress fields
in nonlinear anisotropic viscoelastic laminated composites is
presented and it includes the hygrothermal formulation. Schapery's
single integral formulation is extended to account for anisotropy and
multiaxial stress states. Numerical results obtained from the present
formulation are compared against experimental data and excellent
agreement is obtained between these results. As illustrative examples,
inplane and interlaminar stresses for (45/-45)sT300/5208 laminate are
also presented.
Structural Integrity of Solid Propellants and Filament Wound Rocket
Cases
H. H. Hilton*
DOE Center for Simulation of Advanced Rockets
Nonlinear analytical and massively parallel numerical analyses of
propellants and cases have been initiated. Time-dependent
probabilities of failure will be determined.
Nonlinear Control of Underactuated Systems
V. Coverstone-Carroll,* K. Lee, J. Konicek, S. Coates, J. Hartmann, M.
Zaczek
University of Illinois
Underactuated systems are systems that have more degrees of freedom
than number of actuators. The goal of this research is to extend
existing control theory by designing innovative nonlinear controllers
for underactuated systems and applying them to a three-link
underactuated robot to evaluate the controller performance. This study
will make available new control techniques for underactuated systems.
Also, cost savings will be found in designing and manufacturing
systems with fewer controllers, and systems with partial actuator
failure may still be able to perform their function.
Development and Application of Advanced Feedback Control Design
Techniques for Extreme Maneuverability
P. G. Voulgaris*
U.S. Office of Naval Research Young Investigator Award
The designs of many future military aircraft will incorporate enhanced
maneuver capabilities such as short takeoff and vertical landing as
well as high angle of attack operation. Our objective is to
effectively utilize recent advances in control theory to develop
systematic, efficient, and high-performance design methods for
integrated flight propulsion and high angle of attack control. The
designs will be evaluated and tested in a full-scale nonlinear
simulation. A set of design tools using standard software will be
provided. To accomplish the objective, modern methods of linear robust
and optimal control and nonlinear min-max design, such as nonlinear H
, will be employed.
Robust and High-Performance Control of Multirate Sampled-Data Systems
P. Voulgaris*
National Science Foundation, ECS 93-08481
Multirate feedback systems, like single-rate ones, are sampled data
control systems in which a continuous plant is controlled by a digital
controller using sample and hold devices. Therefore, multirate systems
are hybrid. To obtain a robust and high-performance controller for a
multirate system, it is necessary to consider the hybrid nature of the
problem. In this project a complete methodology of designing optimal H
or 11 controllers for the hybrid multirate sampled data problem is
developed. Also, stability robustness and performance robustness
nonconservative criteria are established. Furthermore, the
asynchronous case (i.e., when not all the rates are related with
rational ratios) is examined. These design techniques can be readily
utilized in aerospace applications.
Reconfigurable Systems for Tailless Fighter Aircraft
P. G. Voulgaris,* J. Medanic, W. R. Perkins, P. R. Kumar, T. Basar
McDonell-Douglas Aerospace (Conducted in the Coordinated Science
Laboratory)
The objective in this project is the development and evaluation of
reconfigurable control algorithms for the new generation of tailless
fighter aircraft. The main effort is placed on the provision of system
identification algorithms that are reliable, fast for on-line
implementation, and efficiently support reconfigurable dynamic
inversion control laws. The overall reconfigurable system is tested
and evaluated on nonreal time simulation as well asreal-time piloted
simulation using the McDonell-Douglas facilities.
Development and Application of Nonlinear Peak-to-Peak Gain
Minimization Control Design for Extreme Maneuverability
P. G. Voulgaris*
U.S. Office of Naval Research, AASERT
The main goal of the proposed research is to further develop and apply
nonlinear controller design methods for supermaneuverable aircraft
using a peak-to-peak gain minimization approach. This type of design
philosophy provides a natural framework for a variety of aircraft
control problems where time-domain and robustness requirements are
imposed on the design in a nonlinear setting. More specifically,
several time-domain requirements can be taken directly into account
such as max overshoot/undershoot specifications, saturation magnitude
and rate limits, etc. Finally, it will complement the nonlinear H
design techniques of the parent grant and provide an additional means
of comparison for nonlinear design.
Bubble Dispersion in Boundary Layers
E. Loth,* K. Felton, T. Bocksell
U.S. Office of Naval Research, N00014-96-1-0312
There is little knowledge of how bubbles react to the turbulence in a
boundary layer. This study is intended to document and understand
bubble dispersion caused by large-scale structures. Cinematography
particle image velocimetry is used to identify velocity vectors along
with bubble velocity throughout a two-dimensional plane of a turbulent
flowfield as a function of time. This is achieved by combining the PIV
technique with a high-resolution movie camera. Various numerical
approximations for describing the turbulent-bubble interactions are
being developed for validation and parameter investigation. These
include continuous random walk descriptions of the turbulent
fluctuations along the bubble path.
Simulation of Icing Clouds and Droplet Impingement on Test Models
E. Loth,* P. Hancir
NASA Lewis Research Center, NAS 3-97011
The objective is to properly predict the uniformity of the liquid
water content produced by spray bars in the NASA Lewis Icing Research
Tunnel used to simulate clouds for icing tests. A computational fluid
dynamics methodology is being developed that treats the droplets in
Lagrangian form and the water vapor in Eulerian form. Re-cent research
has allowed a novel cpu acceleration withindependent local time
stepping for the two referenceframes. In addition, nonlinear
dispersion models are being included.
Aerodynamics
GIGUERE, P. and M. S. SELIG. Freestream velocity measurements for two-dimensional testing with splitter plates. AIAA J., 35:7, 1195-1200 (1997).
GIGUERE, P. and M. S. SELIG. Desirable airfoil characteristics for large variable-speed horizontal axis wind turbines. ASME J. Solar Energy Engr ., 119, 253-260 (1997).
KERHO, M. F. and M. B. BRAGG. Airfoil boundary-layer development and transition with large leading-edge roughness. AIAA J., 35:1, 75-84 (1997).
SAEED, F., M. S. SELIG, and M. B. BRAGG. A design procedure for subscale airfoils with full-scale leading-edges for ice accretion testing. Aircraft, 34:1, 94-100 (1997).
SELIG, M. S. and J. J. GUGLIELMO. High-lift low Reynolds number airfoil design. Aircraft, 34:1, 72-79 (1997).
Astrodynamics
CONWAY, B. A. Optimal low-thrust interception of Earth-crossing asteroids. J. Guidance, Contr., Dyn., 20:5, 995-1002 (1997).
COVERSTONE-CARROLL, V. Near-optimal low-thrust trajectories via micro-genetic algorithms. J. Guidance, Contr., Dyn., 20:1, 196-198 (1997).
WILLIAMS, S. and V. COVERSTONE-CARROLL. Benefits of solar electric propulsion for the next generation of planetary exploration missions. J. Astronaut. Sci., 45:2, 143-159 (1997).
Combustion
BUCKMASTER, J. The effects of radiation on stretched flames. Combust. Theory Modeling, 1,1-11 (1997).
BUCKMASTER, J. Edge-flames. J. Engr. Math., 31, 269-284 (1997).
Computational Fluid Dynamics
CHU, S. S. and K. D. LEE. Euler transonic solutions over finite wings using the Clebsch decomposition method. Int. J. Computat. Fluid Dyn., 8, 209-309 (1997).
EYE, S. and K. D. LEE. Inverse design of transonic airfoils using the Navier-Stokes equations. J. Engr. Optim., 28:4, 245-262 (1997).
FABIGNON, Y., R. A. BEDDINI, and Y. LEE. Analytic evaluation of finite difference methods for compressible direct and large eddy simulations. Aerosp. Res. Technol., 1:6, 413-423 (1997).
LEE, K. D. Changing trends in aircraft design. KSEA Lett., 25:3, 35-41 (1997).
Dynamical Systems
ARNOLD, L., M. DOYLE, and N. SRI NAMACHCHIVAYA. Small noise expansion of moment Lyapunov exponents for two-dimensional systems. J. Dyn. Stability Syst., 12, 187-211 (1997).
DOYLE, M., N. SRI NAMACHCHIVAYA, and H. VAN ROESSEL. Asymptotic stability of structural systems based on Lyapunov exponents and moment Lyaponov exponents. Int. J. Nonlinear Mech., 32:4, 681-693 (1997).
MALHOTRA, N. and N. SRI NAMACHCHIVAYA. Chaotic dynamics of shallow arch structures under 1:1 internal resonance conditions. J. Engr. Mech., 123 :6, 612-619 (1997).
MALHOTRA, N. and N. SRI NAMACHCHIVAYA. Chaotic dynamics of shallow arch structures under 1:2 internal resonance. J. Engr. Mech., 123:6, 620-627 (1997).
McDONALD, R. and N. SRI NAMACHCHIVAYA. Global bifurcations in periodically perturbed gyroscopic systems with application to rotating shafts. Chaos, Solitons, Fractals, 8:4, 613-636 (1997).
Materials and Structures
BUCKMASTER, J. and T. G. VEDERAJAN. Self-heating effects in thermoset composites. J. Compos. Mater., 31:2-21 (1997).
KIM, C. and S. R. WHITE. Constrained warping of thin-walled hollow composite beams. AIAA J., 35:6, 1082-1084 (1997).
KIM, C. and S. R. WHITE. Thick-walled composite beam theory including 3-D elastic effects and torsional warping. Int. J. Solids Struct., 34:31-32, 4237-4259 (1997).
KIM, Y. and S. R. WHITE. Process-induced stress relaxation analysis of AS4/3501-6 laminate. J. Reinforced Plastics Compos., 16, 2-16 (1997).
KIM, Y. K. and S. R. WHITE. Viscoelastic analysis of processing-induced residual stresses in thick composite laminates. Mech. Compos. Mater. Struct., 4, 361-387 (1997).
WHITE, S. R. and A. B. HARTMAN. Effect of cure state on stress relaxation in 3501-6 epoxy resin. J. Engr. Mater. Technol., Trans. ASME, 119, 262-265 (1997).
Propulsion
BUFTON, S. A. and R. L. BURTON. Velocity and temperature measurements in a low-power hydrazine arcjet. J. Propulsion Power, 13:6, 768-774 (1997).
Space Robotics
LEE, K., S. COATES, and V. COVERSTONE-CARROLL. Variable structure control applied to underactuated robots. Robotica, 15:3, 313-318 (1997).
MORRISSEY, J. W. and P. H. GEUBELLE. A numerical scheme for mode III dynamic fracture problems. Int. J. Numer. Meth. Engr., 40, 1181-1196 (1997).
Structural Mechanics
GEUBELLE, P. H. A numerical method for elastic and viscoelastic dynamic fracture problems in homogeneous and bimaterial systems. Compos. Mech., 20:1-2, 20-25 (1997).
GEUBELLE, P. H., M. J. DANYLUK, and H. H. HILTON. Dynamic mode III fracture in viscoelastic media. Int. J. Solids Struct., 35, 761-782 (1997).
HILTON, H. H., S. YI, and M. J. DANYLUK. Probabilistic analysis of delamination onset in linear anisotropic elastic and viscoelastic composite columns. Int. J. Reliability Engr. Syst. Safety, 56, 237-248 (1997).
YI, S., M. F. AHMAD, and H. H. HILTON. Finite element algorithms for dynamic simulations of viscoelastic composite shell structures using conjugated gradient method on vector and coarse grained and massively parallel machines. Int. J. Numer. Meth. Engr., 40, 1857-1875 (1997).
YI, S., and H. H. HILTON. Free edge stresses in elastic and viscoelastic composite laminates under uniaxial extension, bending and twisting. J. Engr. Mater. Technol. Trans. ASME, 119, 266-272 (1997).
YI, S., H. H. HILTON, and M. F. AHMAD. A finite element approach for cure simulation of thermosetting matrix composites. Int. J. Comput. Struct., 64, 383-388 (1997).
Systems and Controls
SALAPAKA, M. V., P. G. VOULGARIS, and M. DAHLEH. SISO controller design to minimize a positive combination of the l1 and the H2 norms. Automatica, 33:3, 387-391 (1997).
SALAPAKA, M.V., M. DAHLEH, and P.G. VOULGARIS. Mixed objective control synthesis: optimal l1/H2 control. SIAM J. Contr. Optim., 35:5, 1672-1789 (1997).
Two-Phase Flow
DEANGELIS B., E. LOTH, D. LANKFORD, and C. S. BARTLETT. Computation of turbulent droplet dispersion for wind tunnel icing tests. AIAA J. Aircraft, 34:2, 213-219 (1997).
LOTH E., M. TAEBI-RAHNI, and G. TRYGGVASON. Deformable bubbles in a free shear layer. Int. J. Multiphase Flow, 23:56, 977-1001 (1997).
LOTH E., S. SIVIER, and J. BAUM. Dusty detonation simulations with adaptive unstructured finite elements. AIAA J, 35:6, 1018-1024 (1997).
OAKLEY T., E. LOTH, and R. ADRIAN. A two-phase cinematic PIV method for bubbly flows. ASME J. Fluids Engr., 119, 707-712 (1997).
Aerodynamics
BRAGG, M. B., S. LEE, and C. M. HENZE. Heat-transfer and free stream turbulence measurements for improvement of the ice accretion physical model. AIAA paper no. 97-0053 (Reno, Nev., Jan. 1997).
GIGUERE, P. and M. S. SELIG. New airfoils for small horizontal-axis wind turbines. AWEA Windpower 1997 Conf. (Austin, Tex., Jun. 1997).
GIGUERE, P. and M. S. SELIG. Aerodynamic blade design methods for horizontal axis wind turbines. 1997 Canadian Wind Energy Conf. (Quebec City, Que., Canada, Oct. 1997).
GOPALARATHNAM, A., M. S. SELIG, and F. HSU. Design of high-lift airfoils for low-aspect ratio wings with endplates. AIAA 15th Appl. Aerodyn. Conf., AIAA paper no. 97-2232 (Atlanta, Ga., Jun. 1997).
JASINSKI, W. J, S. C. NOE, M. S. SELIG, and M. B. BRAGG. Wind turbine performance under icing conditions. ASME/AIAA Joint Wind Energy Symp., AIAA paper no. 97-0977 (Reno, Nev., Jan. 1997).
LYON, C. A., M. S. SELIG, and A. P. BROEREN. Boundary layer trips on airfoils at low Reynolds numbers. AIAA 35th Aerosp. Sci. Mtg., AIAA paper no. 97-0511 (Reno, Nev., Jan. 1997).
MANGE, R. L. and M. B. BRAGG. Unsteady aerodynamics of a chined forebody undergoing forced pitch oscillations. 15th AIAA Appl. Aerodyn. Mtg., AIAA paper no. 97-2211-CP (Atlanta, Ga., Jun. 1997).
NINHAM, C. and M. S. SELIG. An interactive Windows 95/NT version of PROPID for the aerodynamic design of horizontal axis wind turbines. AWEA Windpower 1997 Conf. (Austin, Tex., Jun. 1997).
REICHHOLD, J. D. and M. B. BRAGG. Experimental determination of the droplet impingement characteristics of a propeller. AIAA 35th Aerosp. Sci. Mtg., AIAA paper no. 97-0179 (Reno, Nev., Jan. 1997).
SAEED, F., M. S. SELIG, and M. B. BRAGG. A hybrid airfoil design method to simulate full-scale ice accretion throughout a given CL-range. AIAA 35th Aerosp. Sci. Mtg., AIAA paper no. 97-0054 (Reno, Nev., Jan. 1997).
TANGLER, J. L. and M. S. SELIG. An evaluation of an empirical model for stall delay due to rotation for HAWTs. AWEA Windpower 1997 Conf. (Austin, Tex., Jun. 1997).
Astrodynamics
CONWAY, B. A. and J-J. F. CHEN. Optimal maneuver sequence for Koreasat 1 recovery. AAS/AIAA Space Flight Mech. Conf., AAS paper no. 97-160 (Huntsville, Ala., 1997).
CONWAY, B. A. Optimal interception and deflection of Earth-orbit-crossing asteroids. NATO Advanced Study Institute Conf. on the Dyn. of Small Bodies in the Solar Syst. (Maratea, Italy, 1997).
PRUSSING, J. E. Optimal spacecraft trajectories. Korea-U.S. Wkshp. on Aerosp. Sci.. (Vienna, Va., May 1997).
PRUSSING, J. E. Terminal maneuver for an optimal cooperative impulsive rendezvous. Adv. in the Astronaut. Sci., AAS / AIAA Astrodyn. Conf., AAS paper no. 97-649 (Sun Valley, Id., Aug. 1997).
Combustion
BUCKMASTER, J. Watersheds, ignition fronts, failure waves, catastrophes and inflammability limits. Proc. 16th Int. Colloq. on the Dyn. of Explosions and Reactive Syst., 124-127 (1997).
BUCKMASTER, J. The effects of radiation on stretched flames. 35th Aerosp. Sci. Mtg. and Exhibit, AIAA paper no. 97-0238 (1997).
BUCKMASTER, J. Modeling of microgravity combustion experiments. 4th Int. Microgravity Combust. Wkshp., NASA Conf. Publ. 10194, 355-360 (1997).
BUCKMASTER, J. and P. RONNEY. Flame-ball drift. Proc. 1997 Fall Tech. Mtg., Eastern States Section, Combustion Inst., 427-429 (1997).
BUCKMASTER, J. and J. YAO. Flame configurations in heterogeneous propellants. Proc. 4th Asian-Pacific Int. Symp. on Combust. and Energy Utilization, 112-115 (1997).
BURTON, R. L., D. S. SCHNEIDER, and H. KRIER. Aluminum particle combustion in a pressurized flow reactor. JANNAF 34th Combust. Subcommittee Mtg. (West Palm Beach, Fla., Oct. 1997).
FOELSCHE, R. O., R. L. BURTON, and H. KRIER. Ignition and combustion of boron particles in hydrogen/oxygen explosion products. 35th AIAA Aerosp. Sci. Mtg., AIAA paper no. 97-0127 (Reno, Nev., Jan. 1997).
KRIER, H., R. L. BURTON, M. J. SPALDING, and T. J. ROOD. Study of combustion kinetics of boron particles at high pressure. JANNAF 34th Combust. Subcommittee Mtg. (West Palm Beach, Fla., Oct. 1997).
ROOD, T. J., M. J. SPALDING, H. KRIER, and R. L. BURTON. Ignition dynamics of boron particles in a shock tube. 33rd AIAA/SAE/ASME/ASEE Joint Propul. Conf., AIAA paper no. 97-3234 (Seattle, Wash., Jul. 1997).
SPALDING, M. J., H. KRIER, and R. L. BURTON. Emission spectroscopy during ignition of boron particles at high pressure. 35th AIAA Aerosp. Sci. Mtg., AIAA paper no. 97-0119 (Reno, Nev., Jan. 1997).
VEDERAJAN, G. and J. BUCKMASTER. Premixed flames with edges--failure waves and ignition waves. Proc. 16th Int. Colloq. on the Dyn. of Explosions and Reactive Syst., 132-135 (1997).
VEDERAJAN, G. and J. BUCKMASTER. Premixed flames with edges--failure waves and ingnition waves. Proc. 1997 Fall Tech. Mtg., Central States Section, Combustion Inst., 260-263 (1997).
Computational Fluid Dynamics
BEDDINI, R. A., Y. LEE, and Y. FABIGNON. Estimation of convection errors of finite difference methods for compressible turbulence simulations. FEDSM97-3116, 3rd Symp. on Transitional and Turbulent Compressible Flows, ASME Fluids Engr. Div. Summer Mtg. (Vancouver, B. C., Canada, Jun. 1996).
BEDDINI, R. A., Y. LEE, and Y. FABIGNON. Effects of convection errors on compressible turbulence simulations. Proc. 1st AFOSR Int. Conf. on DNS/LES (Ruston, La., Aug. 1997) 563-570 (1997).
CHAND, K. K. and K. D. LEE. Grid quality improvement via redistribution and embedding. Proc. 15th IMACS World Congr. on Scientific Computat., Modelling and Appl. Math. (Berlin, Germany, August 1997) 2, 241-246 (1997).
EYI, S., M. DAMODARAN, and K. D. LEE. Inverse design of transonic airfoils. 7th Asian Congr. of Fluid Mech. (Madras, India, Dec. 1997).
EYI, S. and K. D. LEE. Transonic turbomachinery blade design using optimization. AIAA 35th Aerosp. Sci. Mtg.,