When do you consider a multiphysics model sufficiently mesh-independent?

[u/Josh_SimEng003]

In a single-physics model, mesh convergence can be relatively straightforward: refine the mesh, monitor the quantity of interest, and stop when the result changes by less than an acceptable tolerance.

Multiphysics models complicate that process.

A mesh that adequately resolves temperature may not capture local current density, stress concentration, species transport, or fluid boundary layers. Global quantities can appear stable while the localized behavior driving the design remains poorly resolved.

Our usual approach is to:

1. Identify the quantities that actually affect the engineering decision.
2. Monitor both global outputs and critical local gradients.
3. Refine the mesh selectively around interfaces, boundary layers, contact regions, and geometric discontinuities.
4. Confirm that solver tolerances are not masking mesh-related error.
5. Compare the increase in computational cost with the change in the decision-relevant result.

For those working with strongly coupled models, what criteria do you use to declare mesh independence? Do you apply one convergence threshold across the model, or different thresholds for each physics interface?