Setting Under-Relaxation Factors

The following guidelines may be helpful for setting the under-relaxation parameters for the velocity solver and the pressure solver:

The default pressure and velocity under-relaxation factors are at fairly optimal settings for steady flow simulations on good quality meshes.

It is a generally accepted rule of thumb that under-relaxation factors for velocity and pressure should add up to 1.

If the residuals are converging quite well it is acceptable to try increasing the under-relaxation factor for velocity slightly (say to 0.8), and decreasing the pressure under-relaxation factor to 0.2.

For compressible flows, it might be necessary to decrease the under-relaxation factor for velocity to 0.5 and possibly the pressure under-relaxation factor to 0.2.

For porous media, it might be necessary to decrease the under-relaxation factor for velocity to 0.5 and possibly the pressure under-relaxation factor to 0.2.

For transient simulations, provided a reasonably well-resolved time-step is chosen, it may be possible to increase the under-relaxation factor for velocity as high as 0.95, simultaneously decreasing the pressure under-relaxation factor to 0.05.

To set the under-relaxation factor for either the pressure solver or the velocity solver, select the node for that solver under the Segregated Flow node in the Solvers node as shown below. The Under-Relaxation Factor property can be entered in the Properties window.