Relaxation

numerics.relaxation_factors controls how strongly each new equation solution changes the current SIMPLE state. A value of 1 applies no under-relaxation; smaller values retain more of the previous state. The aim is to use the largest factor that keeps the coupled solve stable. A factor that is too high for one equation can make that field oscillate and prevent the other fields from converging; reducing every factor more than necessary makes the whole solve slower.

momentum, energy, tke, and sdr use implicit equation under-relaxation. Smaller factors increase the equations’ diagonal dominance, making their linear systems easier to solve. This can make a case more stable, but each SIMPLE iteration makes less progress, so convergence takes longer. These controls act on equations rather than fields, so they are named momentum and energy, not velocity and temperature.

pressure instead uses field relaxation: Vanellus blends the newly solved pressure with the previous pressure before correcting velocity. Smaller factors can similarly suppress iteration-to-iteration oscillation, but slow convergence.

energy relaxation

We strongly recommend setting the energy relaxation factor to 1. Under-relaxing the energy equation moves a large amount of energy into its source terms, effectively storing it between iterations and seriously limiting convergence of the energy balance.

Do not reduce energy relaxation to stabilize a temperature solve. Keep it at 1 and instead use suitable energy-equation preconditioners and the checks in Stabilize the energy solve. Preconditioners and fallbacks explains the available solver controls.

Other relaxation factors

Several API fields use the same term for separate controllers:

• flow_rate_outlet.relaxation_factor damps the outlet-pressure update used to reach its target flow;
• fan_inlet.relaxation_factor and linked_fan_exhaust.relaxation_factor damp the pressure rise read from the current fan operating point;
• a CTM relaxation_factor damps face-temperature updates only with explicit CTM coupling.

These do not replace numerics.relaxation_factors. Adjust the relaxation belonging to the oscillating equation or coupling rather than changing unrelated controls.

See also

• Numerics presets compares the relaxation factors in each standard preset.
• Use stronger numerical relaxation applies these controls to an unstable equation or coupling.
• Residuals and linear-solver convergence explains the outer and inner iterations affected by relaxation.
• Fan and linked-fan boundaries explains fan relaxation.
• Compact thermal models explains CTM coupling relaxation.
• API reference lists exact configuration fields and limits.