Troubleshooting instabilities

Numerical instabilities are a natural and common occurrence in CFD, particularly in strongly coupled or geometrically complex cases. Encountering one does not necessarily mean that the model or solver is broken.

Use these guides when a simulation oscillates, diverges, produces non-physical fields, or fails to settle. The advice is not exhaustive and no strategy will work for every case; it provides starting points for diagnosing the cause and deciding what to try next.

• Diagnose an instability from residuals, linear-solver diagnostics, and monitored field values.
• Correct modeling problems before using numerics to mask an inconsistent setup.
• Switch numerics presets from fast to balanced to stable as the first numerical change.
• Use stronger numerical relaxation for the equation or coupling that oscillates.
• Change preconditioning when a linear system is poorly conditioned or fails.
• Return to first-order advection when a second-order field becomes unstable.
• Stabilize the energy solve without under-relaxing energy except as a last resort.
• Improve mesh quality by controlling aspect ratio, cell transitions, and unnecessary geometric detail.