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 Ed Frulla June 9, 1999 11:28

Mercedes-Benz External Flow Solution using STAR-CD

Mercedes-Benz performed an evaluation of CFD results for the external flow over an E-Series sedan verses the results obtained from a Wind Tunnel.

They used STAR-HPC and a 10,000,000 point mesh.

I would like to obtain a white paper of this experiment for evaluation.

Does anyone know where I can obtain this information?

Was it ever published?

 John C. Chien June 9, 1999 13:16

Re: Mercedes-Benz External Flow Solution using STAR-CD

(1). Do you expect to see something new? (2). A real car geometry is highly complex. Whether the result is realistic or not depends mainly on the turbulence model used in the computation. (3). But so far, the turbulence modeling for 3-D complex geometry with flow separations is still in research stage. The drag prediction will depend on the accurate prediction of the flow separation locations. It is easier to send e-mail to the car company to locate the information if you think it exists.

 Dr. Hrvoje Jasak June 10, 1999 09:59

Re: Mercedes-Benz External Flow Solution using STAR-CD

I'm sorry, but you're wrong: this calculation has been performed on 2 meshes: one with 1,2 million cells and another with 9 million. The drag coefficient for the fine mesh is within 0.5 % of the measured value, and that is with the standard k-epsilon with wall functions (not bad!). Also, the difference between the two results was considerable: you really need this sort of resolution.

The reference for the paper is:

Reister, Leidig, Bauer, Robertson: Flow around a passanger car comparison of numerical results with measurements, Proceedings of the 29th ISATA Conference, Florence, 1996

(If you contact Computational Dynamics, you might be able to get it there.)

 John C. Chien June 10, 1999 10:44

Re: Mercedes-Benz External Flow Solution using STAR-CD

(1). Thank you for the information. (2). 0.5% of the measured value using the standard k-epsilon with wall functions is very good. I agree with you that a fine mesh is essential to get a good result.

 andy June 10, 1999 12:16

Re: Mercedes-Benz External Flow Solution using STAR-CD

Given the known limitations of the k-e turbulence model for this class of flow, I would tend to view an "accurate" prediction with some suspicion and ask the obvious question: how accurate was the lift coefficient?

 John C. Chien June 10, 1999 13:32

Re: Mercedes-Benz External Flow Solution using STAR-CD

(1). With considerable difference between the result of 1.2 million points mesh and that of 9 million points mesh, one really need another data point to establish the convergence trend. (2). I don't know whether the solution at 15 million points will over shoot the test data or not. Anyway, this is not something one can do easily everyday. (3). I think, it is possible to have good result when the design is good. Formula-1 would be the ultimate candidate to test the turbulence model.

 Peter Dittrich July 3, 1999 16:11

Re: Mercedes-Benz External Flow Solution using STAR-CD

The k-e turbulence model is a crude approximation for closing the numerical solution of the coupled transport equations. To claim that with pure grid refinement an accurate solution of the turbulent flow problem can be achieved is a gread oversimplification of the fundamental turbulence problem.

For example, consider stagnation flow in front of the car. Since the k-e turbulence model scales the fluctuations proportianal to the mean velocity gradients, for stagnation flows the turbulent kinetic energy is greatly underestimated.

As long as there is only a law of the wall model and no near wall turbulent boundary layer model is used, all these solutions must be considered "accidental".

 Dr. Hrvoje Jasak July 6, 1999 04:55

Re: Mercedes-Benz External Flow Solution using STAR-CD