Incompressible Fluid Flow - Convergence Problem (Boundary?)

Hey guys,

I try to keep it minimal here but this is my first time working with CAD and SimScale so for me it is a lot of try and error as you can see in my multiple Simulation runs :slight_smile:

I am working on an airy incompressible steady - state simulation and here’s the project link:,service:SIMULATION,strategy:4

There are a few questions but the biggest ones would be:

Which specific advise would you have for me regarding the convergence?

Can anybody tell me why the simulation run 2 and 4 failed? It seems to be the same error message: „Gauge pressure field started diverging. Please check the mesh quality near the reported location and try refining the mesh. If the problem occured near a boundary, please check the boundary conditions.“

The point described is very near to the Pressure outlet boundary condition so I guess either the boundary definition or the mesh is somehow flawed. If the problem is the mesh then how do I refine it at a specific point?

Then can you tell why pressure and omega are instabil. Is it the same reason? I could also imagine that the wind tunnel is not long enough.

Here are the errors and steps I tried:

Run 2.) Error because of gauge pressure diverging

Run 3.) Changed Pressure fixed value to mean value
—> All Convergence plots look good to me but the residual plot for pressure and omega is not stable. Why? I tried to do more Iterations but it didn’t fix the problem.

Run 4.) Turned potential flow initialization on, decreased relaxation factor for pressure from 0.7 to 0.5, chose Mesh 4 which is more fine but lower quality. (And added probe points)
—> Same Error as Run 2
—> Now all values get very unstable towards the end

If somebody looks into it and finds other simulation conditions which seem off please tell me, I’m excited to learn and thanks already for whatever comes out of this!

Hi @nludwig,

Thanks for posting your question. First opportunity that I see here is about increasing the external flow region, which is crucial to help the convergency. Here you find good reference defining that.
About error message for Run 2 and 4, we built this detailed page to help our community users figuring this out, please take a close look and try to the following steps: Divergence in Simulation: How to Tell When and Where? | SimScale

Please try that and let us know how it goes.



Thanks so much for the answer! So I actually increased the region and the gauge pressure is not diverging anymore but I do still have a problem with the convergence residual plot. Do you have another article which might help or other suggestions?

I created a copy of the project to change it so here is the new project link:

And here is the Plot Picture:

I had that exact convergence problem years ago and I believe that I created my own topics and posts about it. I don’t remember what the resolution was. Maybe you can find them in a search of my posts or topics… I am not even sure what to search them for though…

I’ll take any guess and it helps to know that you have been there and got a solution after all so thank you! I’ll go on the forum post hunt now.

I was refreshing my SimScale brain after a 3yr+ absence by searching my popular old topics when and I found one that may help you investigate your issue (it is not where I remember seeing your exact issue tho :slight_smile: ) : In CFD, how to pre-optimize 'Simulation Control' Start time, End time and Delta t with respect to convergence

EDIT: Ooops you solved it, but I will leave this for others.

Ahh yes I appreciate the effort :smiling_face_with_three_hearts: and it still might help me because my convergence is still not at its finest and also wanted to leave for others that apparently my numerics were changed and it helped to setting them to default! Happy simulating.

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We can only dream of a User Interface that shows us what parameters values are not set to defaults AND at the same time, what the default was :pray:

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What a beautiful summary of a quite possible imagination! Some colour, tooltip info or history info. Simulating is starting to seem like a very artistic process to me and I’m here for it :pinched_fingers:

And as we are here for it. My new convergence problem seems to be the outlet which now looks like this:

I feel like there is something fundamental I am missing so if anybody has seen this or suggestions please let them flow. Its als possible to see the values of certain probe points and there seem to be regions which are more stable than others. Could it be that the area is just too complex because of a lot of sharp edges or would you think it is more of a mesh refinement or boundary condition problem?