A New Approach to the Large-Eddy Simulation of Turbulence
R. D. Moser,* S. Balachandar, R. J. Adrian, J. A. Langford, S. Volker, J. W. Wu, K. T. Christensen
National Science Foundation, CTS 96-16219; National Aeronautics and Space Administration, NGT 2-52229; U.S. Air Force Office of Scientific Research

Large-eddy simulation (LES) is a promising technique for turbulence prediction in which the largest, most energetic turbulent eddies are simulated while the effects of smaller-scale turbulence is modeled. New techniques for performing such simulations are being developed based on the formalism of stochastic estimation and on concepts from chaotic dynamical systems. In this new approach, it is possible to optimize both the subgrid model and the filtering operation by which the large scales are defined. Further, one can determine how accurate an LES can be. This new approach should allow LES to fulfill its great promise as an accurate and reliable engineering prediction tool.

Coagulation and Fragmentation of Aluminum Particles and Ash in Rocket Fuel Combustion
H. Aref,* D. O. Pushkin
DOE Center for Simulation of Advanced Rockets

We are studying equations describing coagulation and fragmentation phenomena with a view to applying the results to the aluminum particles used in solid rocket fuels and the ash particles produced once these particles have burned. Modeling of the size and mass distribution of fuel and ash particles is an important input to the global simulation codes being constructed within the Center for Simulation of Advanced Rockets (CSAR). This research is one element of a broader thrust within the 'particle group' of CSAR.

Color Visualization in a Taylor-Couette Device Using Reflective Flakes
S. T. Thoroddsen*
University of Illinois

We present a novel flow-visualization technique utilizing reflective flakes in combination with color illumination. Three differently colored collimated light beams are used to illuminate the flow, each color being directed from a separate direction. In this way, the color of the light reflected from the flakes gives an indication of the local orientation of the stream surfaces, along which the flakes tend to align. In complex flow fields, the distribution of flakes is invariably rearranged by the motion, thus making the local intensity of reflection depend on both orientation and flake concentration. The color, however, is immune to the local number-density of flakes inside the flow, making quantitative information possible. This technique is demonstrated by visualizing the finer details of the modulated wavy Taylor vortices in a Taylor-Couette device.

Computational Turbulence over Wavy Walls
D. N. Riahi,* T. J. Hanratty* (Chem. Engr.), F. Dong
Mobil Technology Center, National Center for Supercomputing Applications

Direct numerical simulation of turbulent channel flows over moving wavy walls is carried out to understand the structure of turbulence. The effects of the curvature of wavy walls on the formation and growth of Gortler vortices are studied. The computational results are compared with available experimental data.

Convection during Alloy Solidification
D. N. Riahi,* B. S. Okhuysen
University of Illinois; Los Alamos National Laboratory

Nonlinear convection during alloy solidification in a normal or high-gravity binary-alloy melt is investigated. Emphasis is given to examination of the mushy layer near the solidification front. Finite-amplitude effects are studied under certain controlling processes by analytical and computational techniques. The models include the basic physical conditions that are of interest in the field of materials processing.

Direct Initiation of Detonation in a Nonuniform Reactive Atmosphere
M. Short*
University of Illinois

This project is concerned with obtaining a complete description for the initiation of a detonation in a multidimensional nonuniformly perturbed reactive fluid. Various forms of initial hydrodynamic perturbations in temperature, pressure, and velocity are being considered. By a combination of asymptotic and numerical methods, we aim to describe the role that each nonuniformity, which typically acts to induce nonuniform compression and expansion flow rates in the fluid, has on influencing the dynamics of detonation initiation.

Drop Impacts
S. T. Thoroddsen*
University of Illinois

The impact of liquid drops onto fluid layers is of importance in a number of diverse fields, such as erosion due to rain, mixing of nutrients in surface layers of lakes, generation of secondary droplets during combustion in enclosed chambers, spray-coating, and cooling. Here we use high-speed video imaging to investigate the details of the impact process. We are focusing on the speed and direction of ejected droplets, as well as the dynamics associated with the initial contact.

Fluid Mixing in Microfluidic Channels for MEMS Biomolecular Systems
H. Aref,* M. A. Stremler
Defense Advanced Research Projects Agency, AFRL F33615-98-1-2853

Mixing in microchannels for use in MEMS-based biodetector systems is being studied analytically and computationally. The computations utilize a commercial code package CFD-ACE+ developed by CFD Research Corporation in Huntsville, Ala. The main idea being explored is to achieve mixing by using chaotic advection in a channel of sufficiently complex geometry. The investigations are coupled with several others being performed by a team led by Professor D. J. Beebe of Electrical and Computer Engineering and the Beckman Institute. We are also exploring electroosmotic flow in microchannels and how to produce secondary flows that can enhance mixing.

Fluid Physics of Foam Evolution and Flow
H. Aref,* S. T. Thoroddsen, D. L. Vainchtein, J. M. Sullivan (Mathematics)
NASA Microgravity Research Division, NAG3-2122

Foam evolution and foam flow are being studied through a series of numerical and laboratory experiments. On the numerical side, the main thrust of the research is to extend current algorithms to more complex foam evolution and to establish the numerical technology to explore three-dimensional foams at the same level of detail as is possible today for two-dimensional foams. Foam experiments use new, developing optical techniques to study the three-dimensional structure of the foam. As the techniques are refined, the experiments will be extended to include studies of dynamically evolving foam.

Ignition of Solid-Propellant in Rocket Motors
M. Short*
DOE Center for Simulation of Advanced Rockets

This project is concerned with describing the complex multidimensional ignition dynamics of the solid propellant found in a rocket motor, such as the solid rocket booster on the Space Shuttle. We are deriving a fluid model that contains all the essential physical dynamics of the ignition process, including solid-to-gas phase transitions, nonuniform surface pyrolysis, and chemical reaction. The effectiveness of various ignition mechanisms is also being calculated.

Large-Eddy Simulaton of Wake Flows
S. Balachandar,* J. W. Wu
U.S. Air Force Office of Scientific Research; National Science Foundation, CTS-9616219

One concept that looks particularly promising in the simulation of turbulent flows is large-eddy simulation (LES). However, LES is not yet a reliable predictive tool, because there are several limiting factors that still remain to be addressed. There is a concentrated effort within the Theoretical and Applied Mechanics Department directed toward the development of optimal formulations of LES using stochastic and dynamical systems concepts that address these issues. Within this larger effort, we will perform a very large-scale direct numerical simulation (DNS) of flow over a circular cylinder at a Reynolds number on the order of 3000. The DNS results will be used to guide the development of LES filters and models.

Mathematical Theories and Modeling for Turbulence
D. N. Riahi*
University of Illinois

Mathematical theories and modeling techniques are being developed for nonhomogeneous turbulence. The goal is to develop reliable and rational turbulence models that can be used for large-eddy simulations of complex nonhomogeneous turbulence problems.

Microgravity Thermocapillary Effects
D. N. Riahi*
University of Illinois

This project studies thermocapillary effects in a microgravity environment. As a first study, thermocapillary convection during alloy solidification is investigated under an external constraint due to rotation about an inclined axis and in a microgravity space environment. Both theoretical and numerical methods are used to determine optimum rotational conditions under which undesirable effects of thermocapillarity are minimized.

Microscale Fluidic Transport and Mixing
R. J. Adrian,* M. Olsen, K. V. Sharp
Defense Advanced Research Projects Agency, FRL F33615-98-1-2853

Mechanisms for mixing in microscale devices used in biological diagnostics and processing are being studied using microparticle image velocimetry and visualization of scalar products. Flow of particle suspensions in microcapillaries having diameters that approach the size of the particles is being characterized.

Mixing in Microscale Fluidics
R. J. Adrian,* M. Olsen, K. V. Sharp
Defense Advanced Research Projects Agency, FRL F33615-98-1-2853

Mechanisms for mixing in microscale devices used in biological diagnostics and processing are being studied. Methods of enhancing fluid mechanical stirring by chaotic advection and the effects of flow separation and unsteadiness are being compared.

Modeling Compressibility Effects in Turbulence
R. D. Moser,* S. G. Borodai, W.-Y. Kwok
U.S. Air Force Office of Scientific Research, F49620-97-1-0089

Most fundamental work in the modeling of turbulence has been done for incompressible turbulence. In this research, we extend these incompressible models to the compressible case using low-Mach-number asymptotics. Such approximations can be valid even in quite large Mach-number flows because the turbulence in these flows is generally at much lower Mach numbers. Thus, an approximation valid to very large Mach numbers can be obtained. The validity of the analysis is being confirmed by appeal to direct numerical simulations of compressible turbulent flows. This asymptotic analysis will be used to develop a rational technique for applying ideas from incompressible turbulence modeling to compressible flows.

Nonlinear Thermal Convection with Variable Gravity and Properties
D. N. Riahi,* A. Hsui* (Geology) F. Dong
University of Illinois, San Diego Center for Supercomputing

Computational and theoretical investigations of nonlinear thermal convection with variable gravity and properties are carried out to determine the effects of variable gravity and/or variable fluid properties on various flow features, such as flow structure, heat flux and nonlinear properties. These studies are important for engineering and geophysical applications. Presently investigations are restricted to planar layers but will be extended later to spherical geometries.

Particle Dispersion in Large-Eddy Simulation
S. Balachandar,* J. Ferry
DOE Center for Advanced Rocket Simulation

A next-generation large-eddy simulation (LES) formulation will be used to evaluate the deterministic predictabilty of advection and dispersion of droplets and particles. Existing subgrid-scale advection and dispersion models will be tested for their performance. Corresponding direct numerical simulations will be performed to guide development of improved subgrid models. Particular attention will be paid to the development of accurate methodologies for tracking particle size and number density spectra within the context of LES. Efficient Eulerian techniques for two-phase flows in the limit of small particle sizes will be developed and compared with accurate Lagrangian results.

Segregation during Flows of Granular Media
E. Fried*
University of Illinois

Applications in which granular media are encountered occur in many industries, including those that involve the processing of minerals, chemicals, pharmaceuticals, and foodstuffs. Quite often the media encountered in such settings are heterogeneous mixtures composed of particles that differ in size, shape, and other characteristics. Segregation, by particle type, commonly occurs during flows of such mixtures, with the influence of particle size believed to be the most significant. Whereas segregation is essential and desirable in mineral processing, it is highly undesirable in chemical and pharmaceutical processing, where the goal is to achieve uniform mixing. The aim of this research is to increase our understanding of the coupled influences exerted by diffusion and convection during segregation by particle size.

Simulation of Aluminum Droplet Combustion
S. Balachandar,* P. Bagchi
DOE Center for Simulation of Advanced Rockets

Detailed 3-D microsimulations of aluminum droplet combustion will be performed. Effects of droplet Reynolds number and critical parameters describing the surrounding medium, such as temperature, species concentration, and local flow gradient will be investigated. A low-Reynolds-number assumption will be made to evaluate flow within the droplet; surface-tension effects of temperature and concentration gradient within the droplet can also be accounted for. The effect of radiative heat transfer on droplet temperature distribution and hence on combustion rate will also be included in the analysis. The present investigation will begin with simple semiempirical chemistry.

Structure of Large-Scale Turbulence in Wall Layers
R. J. Adrian,* S. E. Hommema, K. T. Christensen
National Science Foundation, ATM 95-22662

The structured turbulent eddies in boundary layers and pipe flow are being studied experimentally. Hairpins are found to occur in packets that create low-momentum regions of great streamwise extent. Visualization experiments in the very high Reynolds number condition of the atmospheric boundary layer demonstrate the occurrence of the same mechanism.

Surface Corrugation Effects on Shear Flow Instabilities
D. N. Riahi,* S. Yoo
University of Illinois; National Center for Supercomputing Applications

Effects of moving surface corrugation of arbitrary shape on shear flow structures and instabilities are studied by analytical and computational methods. Certain conditions are determined under which the preferred flow structure is controlled and the flow stability is enhanced by the surface corrugation effects.

Thermochemical Connection with Crystallization
D. N. Riahi,* A. Hsui* (Geology)
University of Illinois

This project involves thermochemical convection in a melt during crystallization, which is of interest in both engineering and geological applications. Engineering applications include casting and crystal growth, while geological applications include magma chamber formation and partial freezing of the Earth's inner core boundary. Presently a stability analysis of particular crystallization models is found to be valuable for understanding the real features of the problems in applications.

Turbulence Simulation for Solid Rocket Applications
R. D. Moser,* R. J. Adrian, S. Balachandar, S. Volker, F. Najjar, Z. Deng
DOE Center for Simulation of Advanced Rockets

New large-eddy simulation (LES) model and simulation techniques are being developed for use in the simulation of solid rocket motors. Solid rocket flows present interesting challenges to turbulence simulation, including large transpiration, combustion, large temperature and density variations, and burning particles. These complications will be treated using the optimum LES modeling technique based on stochastic estimation.

Turbulent Plume Convection
D. N. Riahi*
University of Illinois

Models of cylindrical plumes are being developed for turbulent thermal convection using asymptotic and scaling analyses. These models are based on the restrictions of infinite Prandtl number and steady-state conditions. The models will be extended to arbitrary Prandtl number and unsteady cases and will be compared with the available experimental observations.

Turbulent Thermal Convection
R. J. Adrian,* D. N. Riahi, R. L. Fernandes, R. D. Keane
National Science Foundation, ATM 95-22662

The structure of thermal convection is being studied by particle-image velocimetry techniques in which two-dimensional velocity vector fields in a planar 'slice' of the flow are measured. Of particular interest are the structure at high Rayleigh number and measurement of two-point spatial correlations in convection between hot and cold horizontal plates, convection under a stably stratified layer, and convection over nonuniformly heated horizontal surfaces. The latter is being studied analytically.

Two-Phase Flow in a Hele-Shaw Cell
S. T. Thoroddsen,* E. N. Tan
University of Illinois

The gravity-driven flow of a bubbly liquid between two closely spaced glass plates is studied. Multiple video cameras are used to follow the motion of thousands of bubbles and their interactions. The effective forces of buoyancy, viscosity, surface tension, and inertia depend strongly on bubble size. This dependence leads to the segregation of the different bubble sizes.

Vortex Packets in Turbulent Boundary Layers with Roughness
R. J. Adrian,* S. Balachandar, Z. Liu, C. D. Tomkins
U.S. Office of Naval Research, N00014-99-1-0188

Vortex packets formed by roughness elements bear similarity to those that form on smooth surfaces. They constitute an important mechanism for transport of momentum in either case. The formation, evolution and interaction of packets are being studied by numerical simulation and experimental measurements of the velocity fields using PIV.

Vortex Shedding and Heat Transfer Enhancement
S. Balachandar,* S. J. Parker
NASA Langley Research Center, NAG 1-1583

Recent research has shown that vortex shedding off fins can play an important role in heat-transfer enhancement. This research plans to pursue this idea and address: (1) why compact vortices form at the leading edges of the fin and travel down the fin surface, while such a mechanism is apparently absent in the case of louvers; (2) how to promote vortex shedding in louvered fins; and (3) how fin length, width, pitch, and angle influence vortex shedding.