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Old   December 24, 2008, 10:44
Default Solution not converging - urgent
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Hello Fluent users,

I am analysing 2D airfoil flow at Re = 1 million. When I was using fully turbulent models such as k-e and k-w models the convergence is good but the Cl and Cd results are higher than the experimental results (around 30 % error).

Since I know the transition point I divided the domain into two and assigned fluid zone before the transition as laminar and after the transition point as turbulent.

When I select the laminar zone in the fluid (before transition point) boundary condition my solution is not converging and the residuals are oscillating like small waves especially continuity and epsilon. But the Cd and Cl values are stabilized and close to the experimental values.

Currently I am using realizable k-e model with enhanced wall function and using quick discretization schemes.

I am looking forward for someones advice.

Cheers, Alex

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Old   December 27, 2008, 01:53
Default Re: Solution not converging - urgent
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if the oscillating range is relatively lower and some characteristic value is stabe, u can think u have got a convergence result
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Old   December 27, 2008, 06:28
Default Re: Solution not converging - urgent
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Usually oscillating residuals means the solver is struggling to achieve good convergence...however you are seeing good correlation of the results. I would say that there are some time dependant flow structures due to the turbulant transition that you have included in your simulation. This unsteady flow structures cannot be predicted using the steady state solver, which causes the oscillatory behaviour for the residuals. Quantitatively, some parameters show good results but the overall flow structures may not have converged properly.

I would recommend changing your analysis to unsteady state, and run your case with a low time step. Monitor some parameters and wait until they stabilise. You should get overall good convergence and improved quality for your results.

Hope that helps.
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Old   December 27, 2008, 08:42
Default Re: Solution not converging - urgent
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Thanks Zhaowg and Zongtwi for your kind reply.

I will try unsteady analysis as suggested by Zongtwi and i'll post the outcome.

Further more I need some information regarding steady state analysis. Is it possible to get a converged solution by changing the turbulence source terms in laminar or turbulence zone boundary condition in the form of UDF or constant value.

I changed the wall boundary condition (laminar zone) from no slip to specified shear stress and input the values obtained from contours of wall shear stress from the non converged solution. In this case the convergence is good but the Cl and Cd results are away form the experimental results. i am not sure I am doing in the right way or not.

Your advice is welcome on this issue.

Cheers, Alex.
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Old   December 29, 2008, 07:29
Default Re: Solution not converging - urgent
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I tried unsteady analysis, in this also I am facing the oscillating residuals. The Cl and Cd results are good (less than 5% error).

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