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-   -   High Cd value in FLUENT.. (http://www.cfd-online.com/Forums/fluent/84826-high-cd-value-fluent.html)

John222 February 9, 2011 11:54

High Cd value in FLUENT..
 
3 Attachment(s)
I have done a well mesh in workbench and run the solution but sadly the value of the drag is too high and I guess the problem is with the URF settings, I have uploaded a picture of the solution while converging and a picture of the settings of the URF.

My Cd is supposed to be around 0.4-0.8 not more or less.

In the first I tried the momentum with 0.7 and the solution wasn't stable and got divergence error for momentum then I changed it to 0.2 where I got low coefficient drag force.

Please help me I really need to solve this problem and get a full converged answer, I also uploaded a picture of the mesh for you just in case.


Thanks, John

-mAx- February 10, 2011 02:52

did you set well the reference values?
Is your model scaled?

John222 February 10, 2011 10:43

Quote:

Originally Posted by -mAx- (Post 294581)
did you set well the reference values?
Is your model scaled?

no max I didn't set the reference values its default, and by scaled you mean meshed or what?

-mAx- February 10, 2011 10:55

*with scaled, I meant your model has been designed in mm, and your solver is still in m.
I don't think it should be the case in the new release of Fluent-Ansys, but with Gambit-Fluent combination, you had to scaled your mesh within Fluent (Gambit export a dimensionless mesh)

*Coefficients are Forces divided by 0.5*rho*A*v**2 (where rho, A and v are reference values you have to give in Reference Values Panel)
You don't need to recompute your model since your force is the only value computed within iterations. You just have to adjust the 1/(0.5*rho*A*v**2) factor

John222 February 11, 2011 10:17

Quote:

Originally Posted by -mAx- (Post 294677)
*with scaled, I meant your model has been designed in mm, and your solver is still in m.
I don't think it should be the case in the new release of Fluent-Ansys, but with Gambit-Fluent combination, you had to scaled your mesh within Fluent (Gambit export a dimensionless mesh)

*Coefficients are Forces divided by 0.5*rho*A*v**2 (where rho, A and v are reference values you have to give in Reference Values Panel)
You don't need to recompute your model since your force is the only value computed within iterations. You just have to adjust the 1/(0.5*rho*A*v**2) factor

Have you got any idea how to change the scale cause I think my scale is wrong.

-mAx- February 11, 2011 10:42

I was talking about scale because earlier (fluent 6.3) we had to scale the mesh. Now I assume you don't have anymore to scale your model since you can build, mesh and compute your model within the same soft legacy.
Should be confirmed from someone else.
To check I would compute le massflow through your inlet. You can display the velocity contour on inlet, just for having an idea of inlet velocity (average).
Then Q = U. A, and you get the computed inlet area (for a comparison)


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