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VWT: drag Coefficient

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Hello,


i was trying to get the wright parameter combination of the virtual wind tunnel simulation so that i get the exact theoretical drag coefficient of simple shapes. So i used a sphere with a unit radius r=1 but unfortunately i get a completely wrong value in the end report generated by VWT. So how i get the wright parameters to get plausible results.  By the way a theoretical drag coefficient of sphere should be equal to 0.47


 


in the attachements:


 generated report +screenshots of simulation settings.


 


I would be so glad if you could help me.


 


thanks in advance


 


Regards


 


m:slm 


post-28936-0-08676900-1392277721_thumb.j

post-28936-0-83985700-1392277727_thumb.j

vwtAnalysis.pdf

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1. The mesh shown in the report PDF is very coarse (surface, downstream/wake region), there are no boundary layers.  Try using one of the finer mesh settings, with boundary layers, and with a refinement zone to capture the wake better.


2. VWT was specifically designed for automotive external aerodynamics.  It uses a certain percentage of the bounding box 'front' to calculate the 'frontal area' in the drag coefficient calculation.  This percentage is probably not correct for the sphere.  Check the Frontal Reference Area (A_ref) in table 2 of the report.  ***I believe there is a location in VWT where the user can specify the frontal area to be used for the drag coefficient calculation rather than using the defult.


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  • You have a pretty rough transition between boundary layer and surrounding tetra mesh. Increase the number of boundary layers

The volume mesh in the refinement zones looks coarse, try to refine it.

the frontal area computed in VWT is only an approximation. For the sphere you can compute is analytically and use that value

50m/s inflow speed is a lot. Maybe the result you are comparing to is for a different Reynolds number.

I ran a sphere with 0.1m diameter and inflow speed of 1m/s resulting in Re-number of 7000. The results I got matched very well the experimental data. Attached is the case

Sphere_tutorial.zip

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