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-   -   Divergence from pressure outlet (http://www.cfd-online.com/Forums/fluent/105804-divergence-pressure-outlet.html)

Ananthakrishnan August 9, 2012 08:58

Divergence from pressure outlet
 
Hi,
I am analysing a structured mesh with spalart alamaras model with 1% turbulence level. The quality in ICEM is >0.45 but the residuals in fluent start diverging beginning from the pressure outlet..

Can anyone share some thoughts upon the reasons!!!!

RodriguezFatz August 9, 2012 09:16

Transient or steady state? What kind of solution methods (discretization schemes) do you use? Did you try to switch all of them to standard / (1st order upwind if possible)?

Ananthakrishnan August 9, 2012 13:10

Its steady state..everything is first order.. no change.. It goes on till some 500 iterations.. I also need to run it in second order scheme.. so if i try this solution with second order from 501th iteration it diverges even more faster (which is quite logical)...

RodriguezFatz August 9, 2012 15:16

OK, give some more information: 2d or 3d? Compressible gas? One pressure outlet and one velocity inlet? Any walls? How do you initialize? What happens in the first 500 iterations with your residuals? Any additional models, like energy, multiphase? Do you use SIMPLE for solving?

... you could lower the under-relaxation factors.

Ananthakrishnan August 9, 2012 16:05

http://img521.imageshack.us/img521/1714/fluent.png

i have attached the picture of the residuals.. Its a 3d scalloped flipper wing

http://www.cfd-online.com/Forums/ans...-geometry.html
If you wish you can see the geometry of the wing in the above link.. but the mesh is not the one in the link.. the latest one is a completely structured mesh..

The mach number is around 0.2.. So i am using incompressible flow..out of the six sides of the flow domain, the two on left and right side have been set as symmetry. front side, top and bottom are velocity inlet..no additional models and its just SIMPLE..

hope you get some idea from these info and thanks a lot for your help..

Far August 10, 2012 07:32

can you please show the residuals with latest mesh? Did you try the solution streeing for your case (if it is available for pressure based solver)?

Far August 11, 2012 05:09

Could you please show us the comparison of old and new mesh on convergence ...

Ananthakrishnan August 11, 2012 05:24

http://img138.imageshack.us/img138/8200/transient.png

This is the residuals of the latest mesh.. I had started the transient simulation, thats why you see the huge variations near the end. pls ignore that.

In the second order, the residuals initally decrease, later they neither decrease nor increase!!

Far August 11, 2012 05:37

did you give some special consideration to mesh for using DES? or same mesh as used in RANS?

Ananthakrishnan August 11, 2012 05:39

its the same mesh. I run it in RANS initially. Later when it has converged, i switch to URANS. If you are concerned about the peaks occurring after 800 iterations, then dont be.. because its normal in transient simulation.

Far August 11, 2012 06:00

do you think the quality of mesh (min angle in this case) has the detrimental effect in convergence?

Ananthakrishnan August 11, 2012 06:30

After you increased the min angle to 24 the convergence improved hugely. before at 17 degree as you see even the first order was not converging..
Is it possible to increase the min angle further.. i am trying from last night but i am not getting any big difference..

Far August 11, 2012 06:33

it is restricted by the angle at the sharp trailing.

Ananthakrishnan August 11, 2012 06:40

yeah, i saw it.. but since it is in the flow region (adjacent to the wing) i guess it should not be a serious problem.. so i am ignoring it and trying to improve in the far field region, in the boundary. do you think something like this is possible or is it interrelated!!!


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