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-   -   Convergence help (http://www.cfd-online.com/Forums/fluent/104000-convergence-help.html)

kruno_88 July 1, 2012 07:58

Convergence help
 
Hi,

I'm running a steady flow simulations in Fluent. I'm using default settings for monitoring residuals (0,001) and my simulation convergates at 300 iterations and I get lift coefficient Cl around 4. I run same simulation again just put residuals to stop when decreased at 0,0001 and my solution convergates at 1600 iterations and I get lift coefficient around 8.... and I'm watchnig residuals diagram and in both simulations it looks good. From start it lineary goes down.

Can somebody please explain that big difference? I did that on few simulations and always get about doubled Cl.

Thanks

Jabba July 1, 2012 16:11

Quote:

Originally Posted by kruno_88 (Post 369100)
Hi,

I'm running a steady flow simulations in Fluent. I'm using default settings for monitoring residuals (0,001) and my simulation convergates at 300 iterations and I get lift coefficient Cl around 4. I run same simulation again just put residuals to stop when decreased at 0,0001 and my solution convergates at 1600 iterations and I get lift coefficient around 8.... and I'm watchnig residuals diagram and in both simulations it looks good. From start it lineary goes down.

Can somebody please explain that big difference? I did that on few simulations and always get about doubled Cl.

Thanks

Evaluating only the residuals is not sufficient to trully confirm the convergence of a calculation. You may define monitors to make sure your simualtion reaches a steady state.
In your case, it might be helpful to monitor metrics related to shear stress in the surfaces of interest, as well as lift and drag forces.


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