DES at DLR Experience gained and Problems found K. Weinman, D.Schwamborn

1. DES at DLR Experience gained and Problems found K. Weinman, D.Schwamborn 1

2. Presentation Layout • Introduction • The TAU code • DES Calculations on NACA0012. • DES Calculations on Cylinder • DHT • Conclusions 2

3. Hybrid Navier-Stokes Solver TAU • solution of RANS equations for arbitrary moving bodies on unstructured meshes • independent of grid cell type (hybrid meshes) • state-of-the-art turbulence models SA, SARC, different kω-type models • grid adaptation (refinement & de-refinement) • solver designed for massive parallel computers • computer platform independent • modular library approach • extensions for multi-disciplinary simulations • validated for increasing number of test cases • used by German aeronautical industry 3

4. TAU Applications 32·106 mesh points 4

5. TAU Applications A400M-Sol86: Wing Pressure Distribution TAU Navier-Stokes Calculation 13 Mio. Grid points LSWT Test (Airbus Bremen) Ma = 0.17 Re = 1.3 Mio. Power On Power Off

6. a=17° a=17° a=7° a=7° TAU Applications slat main wing flap a=7° 3D High-Lift Design lift drag Hybrid Mesh, ca. 8.6 Mio points TAU-RANS, SA turbulence model 64 CPUs, Hitachi SR8000 17 h wall clock per point of polar a=17° CL CD a a 6

7. NACA0012 at 60º AoA • Computations are being performed on three meshes. • Mesh C24 : 140 x 60 x 24 (SPTU) • Mesh C48: 140 x 60 x 48 • Mesh F28: 209 x 101 x 29(SPTU) • The Mach number held constant at M = 0.3, but the AoA will be varied according to the following set; AoA(12,45,60) • At present we will discuss results at AoA=60 degrees. 7

8. Lift and Drag Time Series (C24) CL CD Exp.: 0.91 1.65 Coarse(C24) DES : 0.89 1.45 SA: 1.25 2.1 Coarse(C48) preliminary DES : 1.2 1.8 Fine DES(F29)preliminary Δt = .001: 1.24 2.19 Δt = .010: 1.39 2.41 Are high integral values obtained on the finer mesh a consequence of high numerical dissipation ? 8

9. Spectral Analysis of DES Solution (C24) In order to compute a frequency periodogram it was necessary to window the fft in order to reduce spectral leakage. 9

10. Spectral Analysis of RANS Solution (C24) From inspection two frequencies are present , with dominant frequency being O(20) Hz. 10

11. Spectral Analysis of DES Solution (C24) 11

12. Spectral Analysis of RANS Solution (C24) 12

13. Vorticity cuts at different span locations (C24) 141x61x25 point grid x/s = 0.25 x/s = 0.50 13

14. NACA0012 Results on Mesh C48 14

15. NACA0012 Results on Fine Mesh Computations were then performed on the fine mesh. This mesh is, again, an O mesh with the airfoil being descretized into 29 elements in the spanwise (periodic ) direction, 101 cell in the direction normal to the airfoil and 209 cells in the radial direction. 15

16. Vorticity cuts: F29 mesh 209x101x29 point grid x/s = 0.25 x/s = 0.50 16

17. Time Series of Lift and Drag on the Fine Mesh • Influence of time step is clearly seen • Time series length at finer time step was not sufficient for proper unsteady analysis. 17

18. Vorticity iso-surfaces on Fine Mesh The 3D influence in span-wise direction is weak probably due to low span-wise resolution 209x101x29 point grid Z = 0.5c Z = 0.01c 18

19. Flow about Circular Cylinder • First computation performed on structured mesh. The mesh is composed of 6 blocks with 1.18M grid points. • Mesh size: 161x177x41 points • Ma = 0.3 • Re = 10000 • AoA = 0 º 19

20. Time Series: Re = 10000 • The resulting drag agrees quite well with experiments. • Computed integral quantities are as follows: St = 0. 23 Cd = 1.012 20

21. Spectral Analysis: RE = 10000 The spectrogram suggests that significant energy is contained in frequencies up to 50 Hz. The frequency coinciding with the Strouhal number is clearly seen. 21

22. Flow about Circular Cylinder The following film illustrates the vorticity about a cylinder. 22

23. Vorticity Isosurfaces downstream of Cylinder The picture shows the vorticity isosurfaces (computed using the Lambda2 criterion from Hussein) downstream of the cylinder in greater detail. 23

24. Influence of time step size Note that the mean drag value does not appear to be significantly influenced by the choice of time step size, however the resolved spectrum appears to be significantly influenced. 24

25. Decaying Homogeneous Turbulence ( N=32³) Influence of Dissipation Comparison of scalar and matrix dissipation plotted against experimental data. Note that the SA viscosity is still stabilized with scalar dissipation. K(t) ~ t 1.35 25

26. Computational performance • DES is marginally more expensive than properly resolved URANS. • Example: DES on NACA0012 (1.1 GFLOPS on SX5) took about the same time as 2DOF Flutter calculation of NLR7301 (0.9 GFLOPS on SX5) using SA RANS 26

27. Conclusions • DES is computationally demanding, but considerably cheaper than LES or DNS. • DES can give satisfactory results even for dissipative methods. • For compressible codes the dissipation can significantly influence the predicted unsteady behavior. The influence of dissipation can be reduced by a suitable matrix dissipation scheme. • DES is largely suited to flows involving massive separation. Application of DES in its lower limit (sub-optimal LES) requires significant work • in reducing inherent numerical dissipation, • testing for non-reflecting boundary conditions. 27

28. Future Work • Problem in low wave number resolution to be resolved. • Introduction of Preconditioned Matrix Dissipation. • Continuation of current test case studies. • Investigation into H.O.S. • Investigation of other LES/DES variants. 28