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Old   October 17, 2012, 02:26
Default Convergence problems turbo machinary
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baddy
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hi,

i have a modell devided by 4 (90°) my MAX residuals are never under e-02.
coupled rotating domain with stationary domain. air as ideal gas / total energy.
physical time step 0.00001.
interface between domains frozen rotor.

can some one help me to get an convergence?

thanx
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Old   October 17, 2012, 04:08
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Matthias Voß
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export the residuals (CFX-Pre) within the normal result files->solve again starting from your current solution and run just 1 Iter.->visualize the "highest" residuals in CFX-Post as a Isovolume --> i bet you´ll find some mesh issue OR some trans. effect

Is there a part. reason for chossing such a small physical TS?

neewbie
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Old   October 17, 2012, 04:16
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Yes if not such a ts, error comes 'overflow'
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Old   October 17, 2012, 04:27
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tried 'local' timestepping?
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Old   October 17, 2012, 04:29
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thanx for quick answers

no physical ts
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Old   October 17, 2012, 06:46
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what kind of turbomachinery? turbine?

you'vve to provide more information to achieve serious advice.

as it is a stationary simulation, it's not a "real" timestep, but rather a pseudo.

if your mesh has good quality, stat/rot interface is well placed and there were no setup failure (e.g. neglect wall as counter-rotating etc.), try to use a 1st order scheme (also turbulence) or try to use a coarse mesh to achieve convergence.

is there also a diffuser anywhere?!
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Old   October 17, 2012, 06:59
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yes it is a stationary simulation. no turbine. it is a asynchrone machine. i am solving the inner convetive cooling... air produces also heat. my mesh has a good quality but could be better. the geo is very difficult. the inter face i ok ( frozen rotor) no countering wall exist.

Last edited by baddy; October 17, 2012 at 09:13.
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Old   October 17, 2012, 10:33
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can you send an image of your residuals?
are they like a smooth line or not?
are they changing?
and also you should check your results if they are logical residual is not a big deal
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Old   October 18, 2012, 05:16
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Which wall function should be taken? CFX makes it automatically??
what about les ?
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Old   October 18, 2012, 05:54
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Quote:
Originally Posted by baddy View Post
Which wall function should be taken? CFX makes it automatically??
what about les ?
as I mentioned before, it's hard to give you serious advice without any pics.

One for sure: you need a very good mesh for this kind of simulation. y+ should be <1!
that means, that you can't use a wall function. use sst as turbulence model and ensure, that your near-wall boundary layer is resolved accurate. regarding heat transfer issues, you have to be meticulous about that!

y+ values currently?

please provide some pics
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Old   October 18, 2012, 17:18
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Sorry to jump in late but the original question here is an FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   October 19, 2012, 01:55
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ghorrocks i read this time ago. i did not ask before reding this. so i am refinig the mesh.
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Old   October 22, 2012, 05:40
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As I mentioned before (after looking at your residuals), you have problems with energy equation. It's highly proable that you didn't resolve boundary layer accurate and/or use heat model, that doesn't apply to real physics.

But it's difficult to give serious advice without further details.

What kind of mesh do you use? what y+ values do you have? which setup do you use?



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ghorrocks i read this time ago. i did not ask before reding this. so i am refinig the mesh.
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Old   October 23, 2012, 05:03
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my mesh is patially stuctured, partially unstructured. but some geom is nor possible to mesh it with icem (to much time). i do not unterstand the setup? what do you mean with this?

my physical ts is 1*e-6 now is this ok or to small?
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