Troubleshooting CFD Convergence Issues: Tips and Solver Considerations - OPENFOAM

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:

1. 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.

2. 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.

3. 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.

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

5. 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.

6. 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 insights¹².

HAVE YOU TRIED DIFFERENT SOLVERS ?

Yes, trying different solvers can significantly impact the convergence of your CFD simulations. Different solvers have unique strengths and weaknesses, and choosing the right one can depend on the specifics of your problem. Here are a few points to consider:

1. Segregated vs. Coupled Solvers:

   • Segregated Solvers: These solve the governing equations (e.g., momentum, continuity) separately. They are generally more stable and easier to implement but can be slower to converge, especially for complex problems¹.
   • Coupled Solvers: These solve the equations simultaneously, which can lead to faster convergence for certain problems, particularly those involving strong interactions between variables (e.g., pressure and velocity fields)¹.

2. Solver Settings: Adjusting solver settings like relaxation factors, time step size, and convergence criteria can also help. For example, lowering relaxation factors can make the solver more stable but might slow down convergence².

3. Specialized Solvers: Some solvers are designed for specific types of problems, such as multiphase flows or turbulent flows. Using a solver tailored to your problem can improve convergence and accuracy³.

4. Mesh Quality and Refinement: Regardless of the solver, ensuring a high-quality mesh with appropriate refinement in critical areas can improve convergence².

If you haven’t already, you might want to experiment with different solvers and settings to see which combination works best for your specific case.