Symptom: pressure at the interface oscillates after an initially stable ramp.
Likely causes
- Coupling time step too large for the structural response.
- Inconsistent interpolation between fluid and structure meshes.
- Aggressive damping on one side of the coupling.
Tests to run
- Halve the coupling step and track interface residuals.
- Plot transferred forces and check phase lag.
- Temporarily enforce consistent interpolation order on both solvers.
Tip. If you see a clean oscillation frequency, compare it to the first structural mode.