1. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. A Computational Study of the Aerodynamics and Aeroacoustics of a Flatback Airfoil Using Hybrid RANS-LES Christopher Stone Computational Science and Engineering Matthew Barone Sandia National Laboratories C. Eric Lynch and Marilyn J. Smith Georgia Institute of Technology ASME Wind Energy Symposium Orlando, FL 5 January 2009

2. 2009 ASME Wind Energy Symposium Outline Motivation –Flatback airfoils –Goals of the present CFD analysis of flatback airfoils Method –CFD codes and models –Aeroacoustic prediction methods Results –Aerodynamic performance –Vortex-shedding noise –Wake visualization and interpretation Conclusions

3. 2009 ASME Wind Energy Symposium Flatback Airfoils: Background Benefits –Structural benefit of larger sectional area and larger moment of inertia for a given maximum thickness. –Aerodynamic benefits of larger sectional C l max, larger lift curve slope, and reduced aerodynamic sensitivity to leading edge soiling. Drawbacks –Increased drag due to separated base flow. –Introduction of an aerodynamic noise source due to trailing edge vortex shedding. Flatback airfoil shapes have been proposed for the inboard region of wind turbine blades. –Thickness is added about a given camber line – different from “truncated” airfoil. References: C.P. van Dam et al., SAND 2008-2008, SAND 2008-1782, J. Solar Energy Eng., 128:422, 2006.

4. 2009 ASME Wind Energy Symposium Overview of Available CFD Methods The “industry-standard” method for high-Reynolds number CFD is Reynolds-Averaged Navier-Stokes (RANS) –Turbulent fluid flow is modeled using a turbulence model –Typically a steady-state solution is found –Necessary for affordable calculation of thin turbulent boundary layers Drawbacks of RANS –Inaccurate for massively separated flow (flatback wake) –No prediction of unsteady turbulent fluctuations (flatback noise sources) Large Eddy Simulation (LES) –Directly computes the large scale features of a turbulent flow –Models the small-scale, or “sub-grid” features of turbulence Hybrid RANS/LES –Combines RANS in the near-wall turbulent boundary layer region with LES in regions of separated flow

5. 2009 ASME Wind Energy Symposium Goals and Approach Goal: Assess the ability of hybrid RAN/LES methods to: –Predict flatback airfoil lift and drag –Simulate vortex-shedding in the wake for aeroacoustic predictions. Approach –Use multiple CFD codes with hybrid RANS/LES capability –Use measured flatback airfoil shape from Virginia Tech wind tunnel experiment –Run simulations independently on different meshes and using different RANS/LES models (not a rigorous code-to-code comparison) –Compare two different methods for aeroacoustic predictions

6. 2009 ASME Wind Energy Symposium Flatback Airfoil Definition TU-Delft DU97-W-300, 30% thick, 1.5% chord trailing edge thickness DU97-flatback: 10% chord base thickness DU97-flatback with splitter plate

7. 2009 ASME Wind Energy Symposium CFD Codes Multi-block, structured grid finite volume code Stable low-dissipation finite volume scheme 2 nd order implicit time advancement Spalart-Allmaras RANS model Detached Eddy Simulation hybrid model Overset grid finite difference code 4 th order finite difference scheme 2 nd order implicit time advancement Spalart-Allmaras, SST RANS models GT-HRLES hybrid model Unstructured grid finite volume code Node-centered finite volume scheme 2 nd order implicit time advancement SST RANS model GT-HRLES hybrid model OVERFLOWSACCARAFUN3D

8. 2009 ASME Wind Energy Symposium Computational Meshes 3D Domain: Span-wise extent/number of grid points –OVERFLOW: 0.5c with 33 grid points –SACCARA: 0.2c, 0.4c, 0.8c with 64 grid cells –FUN3D: two-dimensional only –Periodic boundary conditions in spanwise direction OVERFLOW SACCARA FUN3D

9. 2009 ASME Wind Energy Symposium Flatback Wake Visualizations 2D DES 3D GT-HRLES

10. 2009 ASME Wind Energy Symposium Aerodynamic Results Chord Reynolds number = 3 million Angle of attack = 4 and 10 degrees Boundary layer transition at x/c = 0.05/0.10 on upper/lower surfaces

11. 2009 ASME Wind Energy Symposium Time-averaged DU97-flatback Lift AOA = 4 deg., All simulations: 0.84 < C L < 0.91 –Experimental C L = 0.81 ± 0.04 AOA = 10 deg., All simulations: 1.56 < C L < 1.66 –Experimental C L = 1.57 ± 0.07 Decent agreement between simulation and experiment Time-averaged lift relatively insensitive to code, grid, spanwise domain size, turbulence model

12. 2009 ASME Wind Energy Symposium Lift Fluctuation Spectrum Experimental acoustic Strouhal number, : St = 0.24 ± 0.01 All simulation results within the range 0.18 < St < 0.23 3D hybrid RANS/LES results within the range 0.18 < St < 0.20 Overflow splitter plate simulation: Strouhal number shifted from 0.20 to 0.28 –Experimental acoustic Strouhal number shifted from 0.24 to 0.30 Overflow, AOA = 4 deg.

13. 2009 ASME Wind Energy Symposium Time-averaged DU97-flatback Drag Experimental free transition C D = 0.06 ± 0.005 Steady RANS: 0.034 < C D < 0.047 Unsteady 2D RANS: 0.052 < C D < 0.144 3D Hybrid RANS/LES: 0.056 < C D < 0.110 Results for drag are sensitive to: code, turbulence model, grid, spanwise domain size Best agreement: FUN3D unsteady RANS (SST model) and Overflow GT-HRLES, C D = 0.055 – 0.056

14. 2009 ASME Wind Energy Symposium Drag from 3D Hybrid RANS/LES 0.2c/64 0.4c/64 0.8c/64 0.5c/33 0.4c/64 0.5c/33

15. 2009 ASME Wind Energy Symposium Effect of Splitter Plate (Overflow results) Splitter plate lengthens wake recirculation zone and decreases intensity of the turbulent Reynolds stress Drag is reduced by 18-27 %, compared to 45-50% reduction observed in experiment

16. 2009 ASME Wind Energy Symposium Aeroacoustic Results

17. 2009 ASME Wind Energy Symposium Aeroacoustic Prediction Methods Computes far-field noise using integration of airfoil surface pressures Sectional lift coefficient spanwise correlation: Correlation lengthCentroid of correlation area 2. PSU WOP-WOP Aeroacoustic Prediction Tool (courtesy of James Erwin, Penn State) 1. Approximate vortex-shedding theory assuming a compact line source imperfectly correlated along the span. L L

18. 2009 ASME Wind Energy Symposium Peak Vortex-Shedding Noise 2D 0.2c/64, 0.4c/64 0.8c/64 0.4c/64 0.5c/33 Simulation results were for tripped b.l.

19. 2009 ASME Wind Energy Symposium Wake Visualization SACCARA, AOA=4 deg. Z=0.8c, Nz=64 C D = 0.084, SPL = 89.9 Z=0.8c, Nz=128 Z=0.4c, Nz=64 C D = 0.11, SPL = 98.6

20. 2009 ASME Wind Energy Symposium Wake Visualization OVERFLOW, AOA=10 deg.

21. 2009 ASME Wind Energy Symposium Conclusions Time-averaged lift insensitive to code, grid, turbulence model, 2D/3D and agreement with experiment is decent Strouhal number is relatively insensitive to the above parameters and is consistently lower than experimental values Overflow simulations predicted qualitative effects of splitter plate: reduction in drag, reduction in noise, increase in shedding frequency Time-averaged drag and noise is sensitive to the above parameters 3D simulations exhibit 2 different vortex-shedding behaviors depending on spanwise domain size and grid resolution –well-defined 2D rollers with streamwise braid vortices : higher drag and louder tone –distorted 2D rollers breaking down into randomly oriented vortices : lower drag and quieter tone More domain size and resolution studies are needed