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-   -   Convergence problems turbo machinary (http://www.cfd-online.com/Forums/cfx/108193-convergence-problems-turbo-machinary.html)

baddy October 17, 2012 02:26

Convergence problems turbo machinary
 
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

mvoss October 17, 2012 04:08

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 youŽll find some mesh issue OR some trans. effect

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

neewbie

baddy October 17, 2012 04:16

Yes if not such a ts, error comes 'overflow'

mvoss October 17, 2012 04:27

tried 'local' timestepping?

baddy October 17, 2012 04:29

thanx :D for quick answers

no physical ts

energy382 October 17, 2012 06:46

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?!

baddy October 17, 2012 06:59

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.

eng_norouzi October 17, 2012 10:33

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

baddy October 18, 2012 05:16

Which wall function should be taken? CFX makes it automatically??
what about les ?

energy382 October 18, 2012 05:54

Quote:

Originally Posted by baddy (Post 387275)
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

ghorrocks October 18, 2012 17:18

Sorry to jump in late but the original question here is an FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

baddy October 19, 2012 01:55

ghorrocks i read this time ago. i did not ask before reding this. so i am refinig the mesh.

energy382 October 22, 2012 05:40

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?



Quote:

Originally Posted by baddy (Post 387440)
ghorrocks i read this time ago. i did not ask before reding this. so i am refinig the mesh.


baddy October 23, 2012 05:03

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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