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Use stronger numerical relaxation

Use stronger numerical relaxation when one outer equation or coupled boundary value oscillates between SIMPLE iterations. It reduces the size of each update and therefore usually slows convergence.

Relaxation is not always a cure. If it is made very strong, a run may appear stable for longer while merely postponing the same failure. Continue checking the residuals and monitored values, and investigate the model, mesh, and linear system rather than treating a calmer history as proof that the cause has been removed.

Choose the factor

You can override relaxation on top of any preset; using stable is not required. Keep the chosen preset as the base and set only the factor being tested:

"numerics": {
  "balanced": {
    "relaxation_factors": {
      "momentum": 0.3
    }
  }
}

Check the base preset’s current value in the API reference, then choose a smaller factor to apply stronger under-relaxation. Change one factor at a time:

  • momentum for oscillating velocity residuals;
  • pressure for oscillating pressure correction or mass balance; and
  • tke or sdr for the corresponding turbulence residual.

Keep energy at 1. Energy under-relaxation is strongly discouraged because it can make energy-balance convergence extremely slow. For temperature instabilities, follow Stabilize the energy solve.

stable already applies the strongest momentum and turbulence relaxation of the standard presets. If it remains unstable, reduce only the factor identified by the iteration history.

Allow more SIMPLE iterations when assessing the change. Do not lower every factor together: that hides which change helped and can make a stable solve unnecessarily slow.

Treat coupling relaxation separately

Fan, flow-rate-outlet, and explicit CTM relaxation factors control their own boundary or coupling updates. Adjust the corresponding API field when that value oscillates rather than changing unrelated equation factors.

See also