DES Workshop, St. Petersburg, July 2./ DES at DLR Experience gained and Problems found K. Weinman, D.Schwamborn

DES Workshop, St. Petersburg, July 2./ Presentation Layout  Introduction  The TAU code  DES Calculations on NACA0012.  DES Calculations on Cylinder  DHT  Conclusions

DES Workshop, St. Petersburg, July 2./  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 Hybrid Navier-Stokes Solver TAU

DES Workshop, St. Petersburg, July 2./ TAU Applications 32·10 6 mesh points

Page 5 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 OnPower Off TAU Applications

3D High-Lift Design  =7°  =17° liftdrag slatmain wingflap  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=7°  =7°  =17° CLCL CDCD  6 TAU Applications

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ Lift and Drag Time Series (C24) CL CD Exp.: Coarse(C24) DES : SA: Coarse(C48) preliminary DES : Fine DES(F29) preliminary Δt =.001: Δt =.010: Are high integral values obtained on the finer mesh a consequence of high numerical dissipation ?

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ From inspection two frequencies are present, with dominant frequency being O(20) Hz. Spectral Analysis of RANS Solution (C24)

DES Workshop, St. Petersburg, July 2./ Spectral Analysis of DES Solution (C24)

DES Workshop, St. Petersburg, July 2./ Spectral Analysis of RANS Solution (C24)

DES Workshop, St. Petersburg, July 2./ x/s = 0.25x/s = 0.50 Vorticity cuts at different span locations (C24) 141x61x25 point grid

DES Workshop, St. Petersburg, July 2./ NACA0012 Results on Mesh C48

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ x/s = 0.25 x/s = 0.50 Vorticity cuts: F29 mesh 209x101x29 point grid

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ Z = 0.01c 209x101x29 point grid Vorticity iso-surfaces on Fine Mesh Z = 0.5c The 3D influence in span-wise direction is weak probably due to low span-wise resolution

DES Workshop, St. Petersburg, July 2./ 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 =  AoA = 0 º

DES Workshop, St. Petersburg, July 2./ Time Series: Re =  The resulting drag agrees quite well with experiments.  Computed integral quantities are as follows: St = Cd = 1.012

DES Workshop, St. Petersburg, July 2./ Spectral Analysis: RE = The spectrogram suggests that significant energy is contained in frequencies up to 50 Hz. The frequency coinciding with the Strouhal number is clearly seen.

DES Workshop, St. Petersburg, July 2./ Flow about Circular Cylinder The following film illustrates the vorticity about a cylinder.

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ 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

DES Workshop, St. Petersburg, July 2./ 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

DES Workshop, St. Petersburg, July 2./ 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.

DES Workshop, St. Petersburg, July 2./ 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.