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For most flows, the default under-relaxation factors do not usually require modification. If unstable or
divergent behavior is observed, however, you need to reduce the under relaxation factors for pressure,
momentum, k, and from their default values to about
0.2, 0.5, 0.5, and 0.5. (It is usually not necessary to reduce the pressure under-relaxation for SIMPLEC.)
In problems where density is strongly coupled with temperature, as in very-high-Rayleigh-number
natural- or mixed-convection flows, it is wise to also under relax the temperature equation and/or
density (i.e., use an under-relaxation factor less than 1.0). Conversely, when temperature is not
coupled with the momentum equations (or when it is weakly coupled), as in flows with constant
density, the under-relaxation factor for temperature can be set to 1.0.

4.3.2 Density-Based Solver Settings

Changing the Courant Number

For ANSYS FLUENT's density-based solver, the main control over the time-stepping scheme is the
Courant number (CFL). The time step is proportional to the CFL, as defined in Equation 18.5-14 in the
separate Theory Guide. Linear stability theory determines a range of permissible values for the CFL
(i.e., the range of values for which a given numerical scheme will remain stable). When you specify a
permissible CFL value, ANSYS FLUENT will compute an appropriate time step. In general, taking
larger time steps leads to faster convergence, so it is advantageous to set the CFL as large as possible
(within the permissible range).

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