Second Symposium on Hybrid RANS-LES Methods, 17/18 June 2007

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2 Zonal-Detached Eddy Simulation of Transonic Buffet on a Civil Aircraft Type Configuration V.BRUNET and S.DECK Applied Aerodynamics Department

3 The Buffet Phenomenon Aircraft in transonic conditions Self-sustained motion Shock wave/turbulent boundary layer interaction Flow separation may appear at high (and low) angles of attack Important pressure (and loads) fluctuations on wings Structure vibrations ( Buffeting ) Limits the flight envelope of the aircraft Limited amount of numerical unsteady studies of the 3D buffet 3

4 2D / 3D buffet 2 D buffet Very important fluctuations Sinusoidal, periodic movement of the shock One main frequency Many URANS/DES/LES numerical studies 3 D buffet Less important fluctuations No main frequency, sometimes a broad frequency band Physics of the buffet very different from 2D s one No numerical unsteady simulations Time history of lift coefficient Time history shock location in one measured section shock location (x/c) t(s) 4

5 Contents Turbulence modeling Test case => wing / body CAT3D model in transonic buffet conditions Description of the computation Grid, numerical methods Results-Discussion Instantaneous, RMS and PSD of pressure signals, global visualizations Conclusions 5

6 Turbulence modeling 2045 LES of a wing ZDES previously validated on a 2D buffet case by Deck (AIAA J., 2005) Detached Eddy Simulation (DES), Spalart et al The needs of different flow regions places conflicting demands on the grid Δtangential> δ in attached BL isotropic LES cells in separation Zonal-Detached Eddy Simulation, Deck 2005 The user selects individual RANS and LES domains grid refinement focused on regions of interest without corrupting the BL properties f vi,w functions disabled in LES mode and Δ=(ΔxΔyΔz) 1/3 ~ ~ 2 ~ υ ~ S d d, ( Δ, Δ Δ ) Δ = max, x y ( d C ) = max DESΔ z 6

7 CAT3D test case CAT3D half wing/body model Expe. by Desprès et al. (2002) in the S2MA wind tunnel of ONERA Mach number = 0.82 AMC = m Re AMC = Angles of attack from 2.2 o to 4.2 o P P P & P Steady and unsteady pressure sensors on different sections of the model Imposed transition at 7% of the chord DSP p' (Pa 2 Hz -1 ) F (H z) Buffet appears at about 3.7 o and is characterized by a broad frequency band 7

8 ZDES grid on the CAT3D model Challenging case for hybrid methods separation line moves in time separation remains close to walls no GIS acceptable Grid patched-grid strategy 84 Blocks for the half wing / body Most of grid nodes are in the separated area and the wake RANS if d w < δ 0 RANS to avoid GIS - δ 0 RANS => δ at shock foot Imposed laminar / turbulent transition at 7% of the chord 8

9 CFD code and numerical methods elsa software (ONERA) Volume cell-centered formulation on multiblock structured grids Spatial scheme Centered second order with artificial dissipation (Jameson scheme) for RANS system of equations Upwind second order scheme (Roe scheme) for turbulent equation Time scheme and implicit phase Second order Gear scheme with 5 Newton sub-iterations (Δt = 10-6 s) Implicit LUSSor on each sub-iteration 9

10 Time history of the simulation 83,000 physical iterations performed => s of physics 2,800 CPU hours on a NEC-SX CD End of transient phase t(s) CL Means RMS and signals acquired 10

11 Global visualization of the computed flow Flow visualization of turbulent field Many turbulence structures are created in the separation, interact with the shock and are convected in the wake 11

12 Time averaged pressure field and skin friction lines Mean Cp P RMS Shock shape and location strongly influenced by the separation Most of the unsteadiness is located in the shock displacement and separated regions Very massive separation Instantaneous skin friction lines 12

13 Time-averaged pressure field Blue : URANS-SA -1 Red : ZDES Cp y / b = 46.66% Significant improvement of the shape of the separated area No GIS with ZDES since any attached BL treated in URANS Separation size slightly overestimated at y/b=73% (deformation effect?) Cp y / b = 60% Cp Cp Cp y / b = 73.33% Cp 13

14 Fluctuating pressure field y / b = 46.66% Prms / Qinf DSP p' (Pa 2 Hz -1 ) Expe. Sec. C ZDES y/b=73.33% x/c=40% Unsteady behavior of the simulation represents quite well the flow physics but overestimates fluctuations Various origins: Insufficient grid density & strategy Wing deformation Prms / Qinf x/c x/c y / b = 73.33% DSP p' (Pa 2 Hz -1 ) F (Hz) Expe.Sec.C ZDES y / b = 73.33% x/c=90% F(Hz) 14

15 Instantaneous field First insight into 3D buffet Pressure waves causing acoustic radiation Streamwise and spanwise propagation Complex flow to be analyzed! Δ.ρU 15

16 Conclusion 3D buffet is a very challenging case for hybrid methods Computation made possible (before 2045!) thanks to the use of a ZDES approach. ZDES allows the reduction of the cost of the simulation compared to a LES one by limiting the extend of the DES zones while maintaining the desired level of accuracy in (U)RANS and focused regions. Significant improvement of the mean field compared to the RANS solution (whatever the grid refinement and the turbulence model): shape of the separated area, pressure levels, shock location Future work Better understanding of the three-dimensional buffet on a civil aircraft configuration Assessment of control devices 16