Simualtion stopped due to the divergence

[Anonymous]

Hello, When doing a wind study for an urban case study, the simulation stopped due to the divergence. I tried several times by changing the settings, e.g. reducing the mesh size, editing CAD geometry to make it simpler, but still facing this problem. Any hint how I can overcome this? It stopped after 250 iterations. I usually set the number of iteration 750, based on guidelines for the wind simulation in Autodesk CFD. Regards, Saeed E

[Jon.Wilde]

Hi,

What happens with just an auto mesh? Typically coarsening the mesh is not so wise (more likely to diverge).

Could you share a little detail about the Boundary Conditions (BC) you are using and perhaps a screenshot also? Is there plenty of clear air downstream?

Thanks,

Jon

[Anonymous]

Hi Jon, Thanks for following up my question. I got the message with both when running with only with auto-mesh and when using a fine mesh region. Boundary box dimensions are approximately 5W, 6L and 5 Hmax. I have 3 times of the model length in the down stream area. I enclosed a screen shot. Regards, Saeed

[Anonymous]

The screen shot

[Jon.Wilde]

Hi,

Either your model is all in white or that upload failed. Might you be able to try again? 🙂

The dimensions do sound OK though - feel free to share a support file (CFZ) also.

Thanks!

Jon

[Anonymous]

I suspected that I wasn't successful with attaching my file. Now I try for the third time to attach files. This time with the support file. 

Regards!

Saeed 

[Jon.Wilde · Accepted solution]

Got it, thanks.

Firstly, the setup looks great - my guess is that the mesh is too coarse. A few thoughts:

We need a minimum of 5-8 elements from top to bottom, here there is just one:

Suppress the buildings:

Try meshing with 3 Gap Elements to better capture the flow between buildings:

Mesh refined to give more elements at the edges:

Don't forget to press 'Refine' to implement the Surface and Gap Refinment before solving.

Use Advection Scheme 5 (Solve -> Solution Controls -> Advection)

The meshing failed at surface 6 - I think it might be worth checking your CAD is clean:

A manual mesh might be a nice quick approach here though. Something like 35m on the air volume and 2m on the building surfaces as a first guess.

It does mean Surface and Gap Refinement have no effect but it also works... 

Hope that helps, have a nice weekend 🙂

Jon

[Anonymous]

Thanks Jon, Sorry for late reflection on your answer. It helped a lot and I could finish the simulation. Kind regards, Saeed