Numerical simulation of the flow in the wake of Ahmed body...

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Numerical simulation of the flow in the wake ofAhmed body using Detached Eddy simulation and

URANS modelling

G. Martinat*, R. Bourguet*, Y. Hoarau**, F. Dehaeze*, B. Jorez*, M. Braza*

* : Institut de mécanique des fluides de Toulouse** : Institut de mécanique des fluides et solides de Strasbourg

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Outline

• Presentation of the Ahmed car body(DESIDER Test case no 4)

• Modelling approaches

• Numerical method

• Results on 25° and 35° slant angle Ahmed car body

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Presentation of the Ahmed car body

• Fully three dimensional flow.

• The flow topology varies with the slant angle :

• Unsteady flow for an angle smaller than 30°

• Steady flow for an angle greater than 30°

left : Lienhart et al, right : Ahmed et al, 1984

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Presentation of the Ahmed car body

• Experimental results from Lienhart et al, 2000 on 35° slant angle geometry and 25° slant angle geometry, providing a steady and an unsteady three dimensional test case

• Ahmed body dimension (Ahmed, 1984) :• Length : L=1m

• Height : h=0.288m

• Front surface : S=0.1120m2

• Reynolds number : Re=768000 based on the height ofthe geometry.

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Modelling approaches

• URANS modelling as a first step

• DES modelling :Use of the model as subgrid model when grid fine enough for LES and as URANS model where grid is not.(Spalart et al, 1997)

The choise of the length scale used is made according :

( )min ,DES RANS DESl l C= ×Δ

• DDES modelling :The aim of DDES is to delay the transition from URANS to LES to avoid having this transition in the shear layer. (Spalart et al, 2006)

Computation have been started, the averaged values are notconverged and won’t be showed here.

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Computations Performed

• 35° slant angle case :• k-ω SST (Menter, 1993)• k-ε Chien (Chien, 1982)• Spalart-Allmaras (Spalart et al, 1992)

• 25° slant angle case :• k-ω SST (Menter, 1993)• k-ω Baseline (Menter, 1993)• Spalart-Allmaras (Spalart et al, 1992)• DES spalart (Spalart et al, 1997)• DDES spalart (Spalart et al, 2006)

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Numerical Method

• Use of the NSMB solver (Currently used by IMFT, CFS Engineering, EPFL, ETH Zurich, IMFS, Dresden and Munich Univ.) Compressible, structured, finite volume, multiblocks, turbulent solver

•Numerical scheme used:•Time scheme : Dual time stepping implicit backwards second order using adaptative time step with constant CFL number.•Space scheme : Central 4th order

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Meshgrid used

• Meshgrid provided by Chalmers-DESIDER consortium• O topology around the body and connectivity with front and wake blocks• 78 blocks and 3 million nodes in full geometry• Full geometry used for DES computations, half geometry used of URANS

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Results for 35° slant angle usingURANS modelling

U velocity profilesk-ω SST and Spalart-Allmaras are giving goodresults

k-ε Chien results don’t fit with the experiment

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W velocity profiles :SST model gives the bestresults

Chien model still does notmatch experiment

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uu and ww velocity profiles :k-w SST better matchs than Spalart Allmaras which can’t capture properly uu and ww

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•SST model overestimates thedetachment of the flow

•k-ω baseline and Spalart-Allmaras models are providingsimilar results whichunderestimate the detachment.

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•SST model overestimates thedetachment of the flow

•k-ω baseline and Spalart-Allmaras models are providingsimilar results whichunderestimate the detachment.

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Conclusion on URANS results

• URANS modelling provides good results for a steady flowexcept for k-ε Chien model

• SST model overpredicts detachment for the 25° case

• Spalart and Menter’s baseline models underpredict detachmentfor the 25° case

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Q criterion on DES and DDES

DDES spalart DES spalart

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Longitudinal vorticity

x=-0.2m x=-0.1m x=0m

x=0.6mx=0.4mx=0.2m

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Longitudinal vortices

x=0m x=0.5m

x=0.5mx=0m

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Results on U velocity profiles

Cx=0.35 for DES and 0.3 for experiment

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Results on W velocity profiles

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Conclusion

• Interesting test case for validation of DES modelling

•URANS modelling provides good results for steady case (concernSST and Spalart)

•DES gives promising results for the unsteady case.

•DDES computations are still in progress to obtain convergedaverage values.

•DES-RANS region is crucial and improvement of statisticalapproach are needed -> advanced URANS approaches (Cf ourpresentations at the IUTAM symposium)

Numerical simulation of the flow in the wake of Ahmed body...

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