Simcenter 3D Solutions Troubleshooting thermal convergence issues and invalid temperature distribution

General guidelines for improving convergence

The following are general steps to take when addressing convergence issues:

• Ensure correct element normals and resolve any geometry problems.
• Ensure thermal couplings are correctly defined and not duplicated.
• Use Deactivation Set Advanced to deactivate sections of the model to identify problematic areas.
• Remove and re-add boundary conditions to identify issues.

Resolving Warning 6209

If you encounter WARNING 6209 during thermal analysis, the following steps may help resolve the issue.

1. Investigate elements: Check the elements mentioned in the warning. The model elements are in the INPF file and thermal solver created elements are in the <simulation/model name>-<solution/analysis name>_report.log file.
2. Improve convergence:
   • Add GPARAM 12 731 -1E36 Card 9 to the solution, if high variation in the conductance values due to the large variation in the material properties or when some conductances are considerably larger than the rest, matrix diagonal rescaling helps with the convergence. For models with PCB stacks, this parameter is particularly effective in aiding convergence.

3. • Avoid using a Perfect Contact type thermal coupling and use a Thermal Coupling with a high Heat Transfer Coefficient of 1e⁵ or 1e⁶ W/m² C when defining a coupling representing perfect contact / zero resistance interface, where the mesh does not match.
   • Reduce the number of thermal elements if possible.
   • Verify values and units interred for the material proprieties, boundary conditions and solution settings.

Conjugate-Gradient solver non-convergence solutions

If you encounter the message indicating that the Conjugate - Gradient solver didn't converge, take the following steps.

• Avoid using a Perfect Contact type thermal coupling and use a Thermal Coupling with a high Heat Transfer Coefficient of 1e⁵ or 1e⁶ W/m² C when defining a coupling representing perfect contact / zero resistance interface, where the mesh does not match.
• Add GPARAM 12 731 -1E36 to the solution, if high variation in the conductance values due to the large variation in the material properties or when some conductances are considerably larger than the rest, matrix diagonal rescaling helps with the convergence. For models with PCB stacks, this parameter is particularly effective in aiding convergence.
• Start at a higher initial preconditioner matrix fill value. For example, if the log file shows that the solver converges later on with a fill value of 200, you can set it directly to this value. If increasing the matrix fill value does not improve convergence, you can increase the iteration limit to 500 or even 1000.

• Use the Ill Condition Filter advanced parameter (Catalog → Thermal Solver). It reduces the number of negative conductances created with the CG method, that helps convergence if there are a lot of 3D elements.

• Remesh the model if there is a poor element quality.

Resolving non-decreasing thermal temperature difference

For models where the thermal temperature difference doesn’t decrease:

• Reduce the relaxation factor, especially if the model contains many temperature-dependent properties.

Addressing invalid temperature distribution

If you have issues with accuracy of temperature distribution, for example problematic situations or spots in temperature for some particular elements, e.g. with low aspect ratios, which are caused by negative conductivities and capacitance distribution coefficients combined with high temperature gradients in certain directions:

• Select Element CG Method from the Element Discretization list and set the Solid Element Capacitance Distribution to At CGs. It lumps the capacitance and elemental heat load at the center of gravity (CG), not at the boundary elements, and creates conductances from CG to boundary elements.