[Gipsea]

Hi there,

I'm new to the cfd computation. I did in the past some FEM work but on mechanical parts

I'm using Freecad for.my design and I'm familiar with freecad wording.
To simplify my question let sayI have 1 box with3 "holes" and the aim is to get the following combination:
1)open constrain with atmospheric pressure
2) inlet to set an air inward speed
3) outlet where I want to consider suction effect. At this stage it can be considered with below atmospheric pressure

With this configuration the computation never converge. On the other hand, If I run computation either only with inlet (outlet @athmospheric pressure) or only with suction (no inlet constrain, only open constrain) converge.

I'd like to share more informations or files if needed but I don't inow which one can be helpful

I'm really struggling at the moment with my project in stand by. I'd really appreciate any hint about where to look for solutions/errors

Thanks

[Martin_Sz]

It sounds like you’re dealing with a challenging CFD problem. Convergence issues can often arise from the way boundary conditions are set up. Here are a few tips that might help you troubleshoot and resolve the issue:

Check Boundary Conditions: Ensure that the boundary conditions are correctly defined. For your setup:

Open Constraint: Set to atmospheric pressure.
Inlet: Define the air inward speed accurately.
Outlet: Set to below atmospheric pressure to simulate suction.
Mesh Quality: Poor mesh quality can lead to convergence issues. Make sure your mesh is fine enough, especially near the boundaries and areas with high gradients.

Initial Conditions: Providing good initial conditions can help the solver converge. Try to start with a simpler case and gradually move to the more complex setup.

Solver Settings: Adjusting solver settings like relaxation factors and convergence criteria can sometimes help. Lowering relaxation factors can make the solver more stable.

Simplify the Problem: As you mentioned, the computation converges when you simplify the problem. This indicates that the issue might be with the interaction between the inlet and outlet conditions. Try to gradually introduce complexity to identify the exact cause.

Check for Physical Realism: Ensure that the physical setup makes sense. Sometimes, unrealistic boundary conditions can cause non-convergence.

If you can share more details or files, it would be easier to provide specific advice. You might also find it helpful to look at resources like this lecture on boundary conditions or this forum discussion for more insights.

HAVE YOU TRIED DIFFERENT SOLVERS ?
MORE AT LINK BELOW
https://howtooansys.blogspot.com/202...ce-issues.html

[Gipsea]

Hi @Martin_SZ and thanks for your reply.
Before all, I'm using OpenFOAM, as add on of Freecad which self generate the files. I'll be more than happy to share the OpenFOAM files if u could possibly suggest which one will help.

After reading your post, to cut off all possible shape influences, I created a pipe with a Venturi neck to easily check the different computation.
With only one inlet (velocity inward) and the outlet (static pressure) the system converge quickly with the expected effect

as second option, I created ashort duct in the ventury neck and set as open (static pressure). from this point onward despite how big or small I set the velocity in and pression on the outlet, system is instable.

I gave a quicl look at your links which are about Ansys (not available for me). Considering the relevance of OpenFOAM I'd like to understand which parameter I should twick.

I used a fine mesh for the whole part

If you are familiar with OpenFOAM please let me know again what file I should share or, considering the simplicity of the model, u might eventually try your own way and share the settings

once again I really appreciate your time and patience
regards