3-D Influence Correction Scheme for Application to 2-D Design
M. S. Selig,* M. D. Soso
Ford Motor Co.

Two-dimensional airfoils designed for race car applications undergo significant performance changes when their low aspect ratio, 3-D counterparts are used in the real situation. In order to compensate for these changes, endplates are attached to both ends of the wings. Research is being carried out to develop a 3-D influence correction scheme that incorporates the effective aspect ratio of the endplates into the initial 2-D airfoil design. This should allow closer matching of the initial design characteristics with the final product.

A Natural Low-Frequency Oscillation on Airfoils near Stall
M. B. Bragg,* A. P. Broeren
NASA Glenn 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.

A Study of Ice Accretion Physics to Improve the Prediction of Ice Accretion on Aircraft
M. B. Bragg,* H. Kim B. Lu
NASA Glenn Research Center, NAG 3-1988

Understanding the relationship between ice accretion geometry and the resulting aerodynamic penalty is important in establishing procedures to determine the most critical ice accretion shape for aircraft certification. This research is the first step in understanding the overall relationships between airfoil geometry, ice accretion, and airfoil performance. Experimental and computational methods are being used. Research is also being conducted under this grant to improve our ability to accurately measure airfoil performance with simulated ice. The presence of the simulated ice causes large regions of unsteady separated flow that make some traditional measurement techniques such as drag wake surveys of questionable accuracy.

Aircraft Icing Research Center
M. B. Bragg,* W. R. Perkins* (Elect. & Comput. Engr.) N. B. Sarter* (Ohio State Univ.)
Campus Critical Research Initiatives Program

Aircraft 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. Our goal is to develop the necessary technology for an aircraft to automatically operate and manage its ice protection system, modify its flight envelope to avoid maneuvers where flight could potentially be uncontrollable, and if necessary, adapt the control system. A human-centered approach will ensure proper pilot-automation coordination. This initiative provides funding for an interdisciplinary team of 9 faculty and their students to initiate research in this area.

Blade Geometry Optimization for the Design of Wind Turbine Rotors
M. S. Selig,* P. Giguere
National Renewable Energy Laboratory, XAF-4-14076-03

A computer program is being developed to facilitate the blade design of horizontal axis wind turbines in which considerations are given to aerodynamics, structures, noise, and cost. Given a set of design requirements and constraints, the program provides optimum blade geometries for minimum cost of energy. To capture the design trade-offs between competing objectives, the program has a multiobjective optimization capability. The program relies on a genetic algorithm-based optimization method and uses the PROPID code for rotor performance analysis. Overall, the proposed method is an efficient engineering tool to design new and retrofit wind turbine blades.

Correcting Inflow Measurements from Wind Turbines
M. S. Selig,* J. Whale
National Renewable Energy Laboratory, XCX-7-16466-0

In order to provide accurate performance data for wind turbine design codes, 3-D field data must be tabulated in terms of sectional angles of attack. A 3-D lifting-surface inflow correction method (LSIM) is being developed using a vortex panel code, to correct the measured local flow angles to angles of attack. The method has been tested using hypothetical 3-D data, based on field measurements from the National Renewable Energy Laboratory (NREL) and wind tunnel data from the Technical University of Delft. LSIM has been used to correct 3-D data from the combined experiment rotor at NREL.

Development of a Method for the Design of Aerodynamically Efficient Juncture Geometries
M. S. Selig,* B. A. Broughton
Private gifts

The design of junctures such as wing/fuselage intersections has traditionally followed a trial-and-error approach, which does not always guarantee an optimal juncture geometry. It is believed that a design method based on a solid understanding of the physics of the juncture flow could lead to significant drag reductions in these areas. The current research is aimed first toward a better understanding of the drag-producing mechanisms in juncture flows through experimental and numerical studies. Second, the knowledge gained will be implemented in an inverse/direct design method to generate highly efficient juncture geometries for a wide range of applications.

Discrete Vortex Simulation to Predict Separation on Airfoils
M. S. Selig,* R. Raju
National Renewable Energy Laboratory, XCX-7-16466-01

In order to predict the performance of a wind turbine in the post-stall region, it is necessary to use airfoil stall characteristics at high angles of attack as input to a 3-D model. A discrete vortex method is being made use of to determine the post-stall properties of airfoils. The method used here is a full vortex cloud method. Discrete vortices are released from every point on the airfoil and convected downstream in the flow field. At high angles of attack, the vortices roll up on the surface of the airfoil, simulating separation that models the post-stall effects.

Effect of Ice Protection Systems on Airfoil Performance
M. B. Bragg,* A. P. Broeren
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,* S. Lee A. P. Broeren
Federal Aviation Administration, DTFA MB 96-6-023

The objective of this research is to study the effect of ice accretion on subsonic aircraft aerodynamics and control. Ice accretion can occur in supercooled large-droplet icing as well as in smaller droplet clouds at temperatures near freezing. Using icing wind tunnels ice accretions are obtained for airfoils with and without ice protection systems. Wind tunnel tests at Illinois and in NASA facilities using simulated ice accretions on an airfoil with a simple flap determine the sensitivity to ice size, shape, and location as well as the role of airfoil geometry. Experimental results to date have explained why certain airfoil designs are more sensitive to these ice accretions.

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.

Hybrid Wing Design to Simulate Full-Scale Ice Accretion
M. S. Selig,* M. B. Bragg,* S. V. Uppuluri
NASA Glenn Research Center, NCC3-509

Aircraft wing ice accretion depends on several factors, but the most important 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 for airfoils (2-D) that has the advantages of full-scale tests without the associated costs. In particular, the full-scale leading edge is tested along with a foreshortened aft section. This methodology is being extended to wings whereby a modified portion of the wing can be tested at conditions that simulate those of the full scale at a fraction of the cost of full-scale testing.

Iced Aircraft Flight Mechanics Modeling
M. Bragg,* S. Lee
STI Inc.

When ice accretes on an aircraft it reduces the lifting capability of the wings, increases the drag and reduces the control authority of the ailerons and elevators. These effects are strong functions of the atmospheric conditions, aircraft design, power and flap settings, etc. Techniques to characterize the effect of ice on the performance and handling qualities of an aircraft are being developed based on dynamic system identification techniques. To develop and test these techniques accurate models of these effects are needed. This research uses computational and experimental techniques (including flight test) to develop accurate aircraft flight mechanics models of aircraft in a wide range of icing conditions.

Low Reynolds Number Airfoil Design and Wind Tunnel Testing
M. S. Selig,* C. A. Carroll, A. P. Broeren
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.

Smart Icing Systems
M. B. Bragg,* N. B. Sarter,* P. G. Voulgaris,* T. Hutchison, J. Merret, R. Oltman, D. Pokhariyal
Campus Critical Research Initiatives Program; NASA Glenn Research Center, NAG3-2135

This part of the larger interdisciplinary/interdepartmental research program addresses the aerodynamic and flight mechanics research required to develop a smart icing system for aircraft. A smart icing system would sense the effect of ice accretion on the aircraft performance and handling qualities and use this information to provide information to the flight crew, operate ice protection systems, provide envelop protection, and possibly adapt the flight control. The research conducted here involves developing robust aerodynamic and flight mechanic models of aircraft in icing conditions, detecting icing through changes in aircraft performance and control inputs, and sensing ice accretion through broad-area aerodynamic sensors. Fundamental research is being conducted in these and other areas in support of the overall Smart Icing Systems Research Program.

Unsteady Flow about an Iced Airfoil
M. B. Bragg,* H. Gurbacki, A. P. Broeren
NASA Glenn Research Center, NGT 3-52357, NAG 3-2271

The prediction of the aerodynamic performance of airfoils and wings with ice accretion is complicated by the unsteady flow that exists near maximum lift. A major feature of this flow is the large separation bubble that occurs aft of the leading-edge ice horn. This experimental study will characterize the unsteady behavior of this bubble using a specially constructed wind tunnel model. High-response pressure transducers, will provide surface pressures and unsteady forces while surface hot films and hot wires in the flowfield will provide information on flow reattachment and details of the vortex structures.

Wind Tunnel Experiments on Meso Flaps for Shock Boundary Layer Interaction
E. Loth,* J. C. Dutton (Mech. & Indus. Engr.) D. Gefroh B. Ford
Defense Advanced Research Projects Agency, F49620-98-1-0490

A novel meso flap system is designed and constructed to relieve the separated flow region caused by oblique shock interaction on a supersonic turbulent boundary layer. Experimental techniques include pressure-sensitive paint, MHz cinematic photography, and LDA measurements. The meso flaps are fabricated from both conventional metal alloys and active smart materials.

Wind Turbine Post-Stall Performance Prediction
M. S. Selig,* N. V. Raj
National Renewable Energy Laboratory, XCX-7-16466-01

The aim of this research is to develop a post-stall model for design and analysis of wind turbines. The aerodynamics of wind turbine blades changes significantly due to 3-D rotational effects on the flow over the blades. The model being developed needs to incorporate these effects to enable accurate prediction of power. The preliminary model was developed, assuming a decrease in drag of the blade. Three key parameters associated with stall delay were identified. Further investigations into experimental and computational results revealed shortcomings in the drag model and an improved model is now nearly complete and is being tested.