Return to first-order advection

If an unstable equation uses second_order_upwind, return that equation to upwind before adding more relaxation.

The standard presets already use first-order upwind. This change is relevant only when the request overrides a preset or uses custom numerics.

Retain the existing preset and replace the affected field, for example:

"numerics": {
  "balanced": {
    "advection_scheme": {
      "momentum": "upwind"
    }
  }
}

Use momentum for velocity instability, energy for temperature, and tke or sdr for the corresponding turbulence field. Change only the field identified by the residual history.

If first-order advection restores stability, keep it while testing the remaining model and mesh changes. The result indicates that the higher-order reconstruction is interacting with unresolved gradients, mesh quality, or the coupled iteration; it does not identify which one by itself.

See also

• Advection schemes explains the two schemes and their stability trade-off.
• Improve mesh quality covers mesh changes before reconsidering a higher-order scheme.