code_saturne User's Forum

Hello,

Now I am trying 3D flow simulation by URANS for simple geometric room.
The room dimensions are about 3m*10m*3m (width, width, hight). More than 2miliom mesh of tetra is used for this room, and 1cm mesh is piut around outlet condition.

However, I always faced a divergence problem.
The divergence always starts from outlet boundary.
For inlet bounday condition, I put 0.1m/s velocity, and simple outlet condion is used for outlet boundary.

When I check "listing" file, I always had messages like "Incoming flow detained for 51 outlet faces on 8350".

I am wondering what kind of setting casues this problem even though I suppose that simple settings applied to this simulaiton.

Would someone give me tips?
I think this simulation should be done easily because it is simple case.

Best regards.
Tsubasa

Hello,

Did you extrude the mesh on at least 1 layer at the outlet so as to have prisms (orthogonal to the outlet) and not tetrahedra there ? This can really help. Also, using an imposed pressure outlet is more robust than a default "free outlet".

There are a few additional recommendations : https://github.com/code-saturne/code_sa ... robustness

Regards,

Yvan

Hello,

Actually, I have to generate only tetra mesh in all region
because I will carry out a postprocessing specialized to tetra mehs result.
That's why I am using tetra mesh only, even at 1 layer at the outlet.

In future, I want impose pressure values at inlet and outlet (pressure driven flow),
but as far as I understand, computations are often unstable when we use tetra.

I tried the recommendation you gave me, but it cannot solve the problem....
so I have to find some solution for this.

Best regards,
Hamada

Hello,

You can still use a hybrid mesh and output only the tetrahedral part (using a selection filter) for postprocessing.

Did you try the other recommended robustness parameters in the wiki ?

Regards,

Yvan

Hello Yvan,

OK, that's is one of the options.

I tried other recommendated setting, but it did not solve the problem.

Best regards.
Hamada

Hello,

How big is your mesh ? If you have a small enough mesh and the computation crashes on it, I am interested if you can provide it so we can use it to test/improve robustness.

Best regards,

Yvan

Hello,

Mesh size is very small. Near the outlet boundary, 2cm tetra mesh is used, and other area has 5cm mesh.

Can you download a mesh file and setup file in here?
https://filesender.renater.fr/?s=downlo ... b0910cc9d4

Although this geometry is not same the picture in first post, this geometry and mesh has a same divergence problem.
DIvergence always comes from outlte boundary condition.
The simulation works very well if I use hexa mesh, but I prefer tetra mesh.

Best regards,
Tsubasa

Hello,

Yes, I was able to download the mesh. I'll try to run tests with it soon. Thanks.

Best regards,

Yvan

Hello Yvan,

Thank you for your help.
I am appreciate if you could let me know your result of testing.

Best regards,
Tsubasa

Hello,

How was your test?
I would appreciate if you let me know.

Best regards,
Tsubasa