26.3.2 Setting Under-Relaxation Factors

The pressure-based solver uses under-relaxation of equations to control the update of computed variables at each iteration (as described in this section in the separate Theory Guide). This means that all equations solved using the pressure-based solver, including the non-coupled equations solved by the density-based solver (turbulence and other scalars, as discussed in this section in the separate Theory Guide), will have under-relaxation factors associated with them.

In ANSYS FLUENT, the default under-relaxation parameters for all variables are set to values that are near optimal for the largest possible number of cases. These values are suitable for many problems, but for some particularly nonlinear problems (e.g., some turbulent flows or high-Rayleigh-number natural-convection problems) it is prudent to reduce the under-relaxation factors initially.

It is good practice to begin a calculation using the default under-relaxation factors. If the residuals continue to increase after the first 4 or 5 iterations, you should reduce the under-relaxation factors.

Occasionally, you may make changes in the under-relaxation factors and resume your calculation, only to find that the residuals begin to increase. This often results from increasing the under-relaxation factors too much. A cautious approach is to save a data file before making any changes to the under-relaxation factors, and to give the solution algorithm a few iterations to adjust to the new parameters. Typically, an increase in the under-relaxation factors brings about a slight increase in the residuals, but these increases usually disappear as the solution progresses. If the residuals jump by a few orders of magnitude, you should consider halting the calculation and returning to the last good data file saved.

Note that viscosity and density are under-relaxed from iteration to iteration. Also, if the enthalpy equation is solved directly instead of the temperature equation (i.e., for non-premixed combustion calculations), the update of temperature based on enthalpy will be under-relaxed. To see the default under-relaxation factors, you can click the Default button in the Solution Controls task page.

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 $\epsilon$ 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.

For other scalar equations (e.g., swirl, species, mixture fraction and variance) the default under-relaxation may be too aggressive for some problems, especially at the start of the calculation. You may wish to reduce the factors to 0.8 to facilitate convergence.

User Inputs

You can modify the under-relaxation factors in the Solution Controls task page (Figure  26.3.1).

Solution Controls

Figure 26.3.1: The Solution Controls Task Page for the Pressure-Based Solver

You can set the under-relaxation factor for each equation in the field next to its name under Under-Relaxation Factors.

If you are using the pressure-based solver, all equations will have an associated under-relaxation factor (see this section in the separate Theory Guide). If you are using the density-based solver, only those equations that are solved sequentially (see this section in the separate Theory Guide) will have under-relaxation factors.

If you change under-relaxation factors, but you then want to return to ANSYS FLUENT's default settings, you can click the Default button.

Note that with optimal settings, the convergence of the coupled pressure-velocity algorithm will be limited by the segregated solution of other scalar equations, e.g., turbulence. For optimum solver performance, you will need to increase the relaxation factors for these equations to a value greater than the default values.