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Old   October 9, 2008, 04:05
Default Transition - no convergence
  #1
Christian
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Hej Folks,

I am getting weird with my simulation. I try to use the Langtry-Menter Turbulence Model to simulate transition on an airfoil. My solution doesn´t converge anyway. I have tried lots of timescales, refined mesh (Y+ = 1 to Y+ = 0.1), but there is no convergence. All my solutions converge to a Residual MAX 10^-3, but my goal is a convergence criterion with MAX 10^-6.

Perhaps you guys have any idea what the problem is. I would be very glad if anyone could help me!!!

Thank you very much in advance!!!!!!
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Old   October 9, 2008, 07:26
Default Re: Transition - no convergence
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Adam
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Hi Christian

Its difficult to tell from your description what the problem is. Try running a range of timesteps, or start with upwind scheme first.

If that does not work, then a turbulent model such as kw SST.

If that doesn't work, then the problem may well be physical rather than numerical, and you may need to readjust the problem you are trying to solve.

Cheers

Adam
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Old   October 9, 2008, 08:42
Default Re: Transition - no convergence
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Christian
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Hej Adam,

thank you very much for the information!! I have a well converged solution with the SST model and a fully turbulent Boundary layer. So I thought if I use that as an initial values file it probably converges. But it doesn´t.

I have tried several timesteps and always used the upwind scheme.

At the moment I am trying the simulation on a very fine grid but it seems not to converge.

What do you mean with "physical problem"? Do you think I should try with higher/lower velocities/Re Numbers?

Thank you very much. It makes me very happy to get help from you!!!

Cheers Christian
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Old   October 9, 2008, 09:31
Default Re: Transition - no convergence
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Adam
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It may be that you have a numerically unsteady transition region,or that the impact of transition has changed the trailing edge characteristics of the wing.

Try running high resolution. Make back-up files at various points of unsteadiness in the residuals and compare the flow and boundary layer. This may give an indication as to why there is unsteadiness in the residuals.

Adam
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Old   October 9, 2008, 18:48
Default Re: Transition - no convergence
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Glenn Horrocks
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Have a look here:

http://www.cfd-online.com/Wiki/Ansys...gence_criteria

I bet the transition and/or the laminar separation bubble is moving so you have a transient simulation.

Glenn Horrocks
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Old   October 13, 2008, 08:08
Default Re: Transition - no convergence
  #6
Christian
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Hej Guys,

thank you very much for your help!!! I now have a converged solution, so I will start a parameter study and check out the influence on the solution.

Thanks a lot again!!!!

Cheers Christian
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Old   April 11, 2011, 07:07
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  #7
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Nick
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Hi Christian!

I don't know if you still visit the forum but would you please comment on what you changed? I'm stuck with the same problem.

Regards,
Nick
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Old   May 12, 2011, 05:25
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iswadi
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me too. i have changed my into sst model. however it seems does not converged at high AOA (> 16 degrees). any suggestion is almost welcome
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