Learning Forum

[617283443]

Hi experts!

When performing numerical simulations in Fluent, how can one determine whether computational divergence stems from an improperly set under Relaxation Factors? If the under Relaxation Factors be set too low, computational time may become excessively prolonged. How should the under Relaxation Factors be appropriately configured to ensure convergence while minimising iteration time?

[zongbo.wei]

The default value is applicable in most cases.You can refer to the following steps to find the optimal value.

• Start with default URFs recommended by Fluent for your physics and solver type (e.g., 0.7–0.8 for pressure in PISO, 0.2–0.5 for VOF, 0.5 for Reynolds stresses in RSM).  

• If divergence occurs, decrease URFs incrementally (e.g., reduce by 0.1–0.2) for the problematic variable(s). For multiphase or mixture models, begin with low URFs (0.2 or less for slip velocity) and increase gradually if convergence is good. 

• Avoid setting URFs too low unless necessary for stability; values below 0.1 should be used only temporarily and increased once the solution stabilizes. 

• For turbulent flows, modifying only the URFs for turbulence variables (rather than all variables) may yield better convergence. 

[617283443]

Thank you for your reply. How am I to know which one is my problematic variable(s)?

[zongbo.wei]

Thank you for your feedback！
For instance, if the pressure residual curve fluctuates greatly or does not decrease, it is recommended to lower the under-relaxation factor of pressure (e.g., to 0.2); if the momentum residual is abnormal, adjust the under-relaxation factor of momentum (e.g., to 0.4 to 0.5); when the residuals of turbulence parameters (such as k, ε or ω) do not converge, reduce their under-relaxation factor to 0.4. When the convergence of the energy equation residuals is difficult, especially in problems of heat conduction or high temperature gradients, the energy under-relaxation factor can be reduced to 0.95 or even lower.