Numerical Investigation of Circulation Control Airfoils Byung-Young Min, Warren Lee Robert Englar, and Lakshmi N. Sankar School of Aerospace Engineering Georgia Institute of Technology, Atlanta, GA, Outline Background Research Objectives Configurations studied Mathematical and Numerical Formulation Results and Correlation with Experiments Effects of formal spatial accuracy Effects of jet turbulence intensity Effects of grid density Effects of the inclusion of plenum and nozzle geometry in the model Effects of turbulence model Conclusions and recommendations Background Noise pollution from the large aircraft has become a major problem that needs to be solved. NASA proposed a plan to reduce the noise by a factor of four (20dB) by A major source of large aircraft airframe noise during take-off and landing is the high-lift system - namely flaps, slats, associated with flap-edges and gaps. The high-lift system also contains many moving parts, which add to the weight of the aircraft, and are costly to build and maintain. These devices for generating high lift are necessary for large aircraft that use existing runways. Boeing 737 Wing/Flap System (Paper by Robert Englar) An alternative to conventional high-lift systems is the Circulation Control Wing (CCW) technology. The CC wing can generate the same high lift with much less complexity compared to the high-lift system, and many noise sources such as flaps and slats, can also be eliminated by the CC wing. For example, as shown in previous figure, there are just 0-3 moving elements per wing for a Circulation Control wing, compared to 15 moving parts of a conventional Boeing 737 wing with high-lift systems. Circulation Control Wing Concept Circulation Control Aerodynamics: In this approach a tangential jet is blown over a highly curved aerodynamic surface (the Coanda surface) to increase or modify the aerodynamic forces and moment with few or no moving surfaces. Figure (Taken from paper by Englar) shows a traditional Circulation Control Airfoil with a rounded trailing edge. At very low momentum coefficients, the tangential blowing will add energy to the slow moving flow near the surface. This will delay or eliminate the separation, and is called Boundary Layer Control. When the momentum coefficient is high, the lift of the wing will be significantly increased. This is called Circulation Control. The lift augmentation, which is defined as C L / C , can exceed 80. Circulation Control Wing Concept In general, the driving parameter of Circulation Control is the jet momentum coefficient, C , which is defined as: Prior Work CCW Airfoil with a Sharp Trailing Edge Prior Work Lift Coefficient vs. C Angle of Attack 0 degrees, Integral Flap at 30 degrees Prior Work Lift Coefficient vs. Angle of Attack Research Objectives Extend a previously developed 2-D Navier-Stokes based approach for CCW airfoils with sharp trailing edge to CCW sections with rounded trailing edge. Assess the effects of several physical and computational parameters on the predictions. Grid density Formal accuracy of the algorithm Turbulence models Detailed representation of the plenum and nozzle geometry Jet turbulence intensity Draw conclusions and make recommendations for future computational experimental studies. Mathematical and Numerical Formulation A 3-D multi-block compressible Navier-Stokes solver is used. 2-D configurations may be modeled as a special case. The inviscid flux derivatives are modeled using 3 rd order, 5 th order, or 7 th order accurate weighted essentially non-oscillatory interpolations. The viscous terms are modeled using standard second order central differences. The equations are solved by marching in time using a temporally first order accurate LU-SGS scheme. Time-accurate modeling as well as local time stepping are available as user-supplied options. A variety of turbulence models are available: Spalart Allmaras (SA) and SA-Detached Eddy Simulation (SA-DES) models Classical - model - / - blended Baseline ( - BSL, Menter model - SST (Menter) model This solver was extensively modeled for AGARD standard test cases (e.g. RAE 2822 supercritical airfoil) prior to its use in the present study. Configuration Being Modeled NCCR N airfoil tested at David W. Taylor Naval Ship R&D Center by J. Abramson in The chord length is cm, with the slot position at 0.967c. A slot height to chord ratio (h/c) of was selected for current study. The freesream dynamic pressure was N/m 2. The freestream static pressure and density were assumed to be pa, and kg/m3, respectively. The corresponding freestream Mach number is calculated as and the Reynolds number is estimated as 5.4510 5. Numerical Results The momentum coefficient was changed over the range to Systematic Studies were done to assess the effects of the following factors on the prediction: grid density formal spatial accuracy jet turbulence intensity inclusion of plenum and nozzle geometry in the model turbulence models Grid Topology Grid Sensitivity and Spatial Accuracy Studies (C = 0.209) Grid 1 Block 1 : 319 50, Block 2 : 368 95 Grid 2 Block 1 : 434 70, Block 2 : 503 97 Accuracy 3 rd order, - SST 7 th order, - SST 3 rd order, - SST 7 th order, - SST CLCL Average wall y For the limited range of grid densities considered, the solution and the Formal accuracy of the solution had minimal influence on the overall loads. 3 rd order, - BSL3 rd order, - SST 7 th order, - SST7 th order, - BSL Eddy viscosity ( C =0.209) Higher Order schemes Resolved the wall jet and the confluent boundary layers more crisply Effects of Inclusion of the Plenum and Nozzle Geometry (spatially 3rd order scheme, - SST, C =0.209) Inclusion of the plenum had relatively small effect on the flow patterns and overall loads Expt With plenum Without Plenum Inlet I TB -20 % Effects of Jet Turbulence Intensity In most experiments, the turbulence intensity level of the jet is not measured. Our studies indicate this is an important parameter and may have a significant effect on the computed solutions. Exp. J. Abramson, 1977 Predictions (3 rd order, SST) Jet intensity -10 %20 % CLCL Effects of Turbulence Models on Evolution of Lift with Time C =0.025C =0.209 Effects of Turbulence Models on Evolution of Lift with Time C =0.025C =0.209 Instantaneous Streamlines at Nominal Steady State or Limit Cycle, C =0.025 (top) and (bottom) - SST - BSL SA-DES - SST - BSL SA-DES Observations on the Adequacy of Turbulence Models Most models (SA-DES, - / - blended, - SST) assume that there are two dominant shear layers and associated length scales. In these models, Region close to the wall has eddies of the size comparable to the distance from the wall Regions away from the wall have length scales comparable to shear layer thickness, or grid size None of these two layer models properly model CCW effects caused by three or more shear layers (wall jet, surface boundary layer upstream of the slot, mixing layers). Among the models tested, the - BSL - / - blended) model performed best. Effects of Turbulence Modeling on Airloads Surface pressure distribution C =0.209C =0.025 Conclusions Reynolds-Averaged Nervier-Stokes simulations have been done for a circulation control airfoil for range of momentum coefficients. The effects of grid density, spatial accuracy, upstream turbulence level at the jet slot, and turbulence modeling have been investigated. It was found that turbulence models dramatically affected the wall jet behavior and its detachment point and hence the overall lift value predicted. The turbulence level at the jet slot was also found to have a noticeable influence on the computed solutions. For the grids used in this study, use of high order spatial accuracy algorithms appeared to achieve an enhanced resolution of the wall jet, boundary layers, and the mixing layer, but had negligible effect on the overall loads. The inclusion of the plenum chamber and the jet nozzle was found to have negligible effect on the overall loads. Among the turbulence models tested, the blended - / - model (referred to as - BSL) performed the best for the entire range of the momentum coefficient considered. Recommendations These conclusions are based on correlations with measured data for the overall lift coefficient for the NCCR airfoil and a sharp-trailing edge airfoil studied previously. For a further assessment of these results, and for improved modeling of the CCW flow phenomena, it is essential that the turbulent flow behavior be characterized through flow visualization and hot wire measurements of the turbulent flow field downstream of the jet slot. Acknowledgements This work was supported by the NASA Langley Research Center under the NASA Grant and Cooperative Research Agreement (NRA) NNX07AB44A. Dr. William E. Milholen is the technical monitor. The authors are thankful to Greg Jones of NASA Langley Research Center for his interest and encouragement throughout this study.