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Erdem March 4, 2006 18:46

3d ahmed body flow
hi everyone. my question is about Ahmed Body. have you ever used this 3D problem for your validation cases. my problem is the material properties of air. my drag coefficient is %30-40 higher than the experimental value. I used ro as 1.14075 kg/m3 (@ h=426m T=20Celcius). the height is the geographical altitude of the Gottingen city that the original experiment conducted. but there is no info about material properties. does anyone interested in this topic?

Jim March 5, 2006 17:47

Re: 3d ahmed body flow
I think you should look elsewhere for the difference between your result and the experiment. My guess is that air at ambient temperature and close to sea level, is pretty regular air. However, the position where the flow separates and re-attaches (at certain backlight angles the separation point is not fixed), the quality of your mesh, the turbulence model used, whether lift and drag are well converged, etc. may combine to produce a 30-40 % difference.

Erdem March 5, 2006 19:26

Re: 3d ahmed body flow
I'm using a 3D model with approx 1million cells. I tried different back angles. I'm using fluent and very accurate results are found with fluent in 2002 with SST and realizable k-e model. the experimental drag coefficient is 10 degree Cd=0.230 I found 0.280 RKE y+>30 standard WF 25 degree Cd=0.285 I found 0.316 RKE y+>30 standard WF 30 degree Cd=0.378 I found 0.345 RKE y+>30 standard WF 35 degree Cd=0.260 I found 0.350 RKE y+>30 standard WF

bound conditions

velocity inlet with 60m/s, Turb In. 0.5%, 3.077m Hydraulic Dia pressure outlet with 0Pa gauge pressure (modeling open test section) , a symmetry plane and the rest is wall of course. The engineering working in fluent distibutor told me there isn't any problem in my model so I tought that the problem is in the material properties or smth else.

I know the physics of ahmed body very well but neither of the RANS models can capture the separating and reattaching of the flow for 25 degree back angle case. LES is too computationally expensive I don't know what to do

there is a pdf document about accurate fluent results that I mentioned above. pages 34-35

William Blake March 6, 2006 05:07

Re: 3d ahmed body flow
Did you run a transient calculation!!

Erdem March 6, 2006 07:02

Re: 3d ahmed body flow
no I'm using steady-segregated node based solver of fluent 6.2

William Blake March 6, 2006 11:05

Re: 3d ahmed body flow
Run it transient and tell us what happens.

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