
As discussed in Section 25.4.4, the pressurebased solver uses underrelaxation of equations to control the update of computed variables at each iteration. This means that all equations solved using the pressurebased solver, including the noncoupled equations solved by the densitybased solver (turbulence and other scalars, as discussed in Section 25.1.2), will have underrelaxation factors associated with them.
In FLUENT, the default underrelaxation 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 highRayleighnumber naturalconvection problems) it is prudent to reduce the underrelaxation factors initially.
It is good practice to begin a calculation using the default underrelaxation factors. If the residuals continue to increase after the first 4 or 5 iterations, you should reduce the underrelaxation factors.
Occasionally, you may make changes in the underrelaxation factors and resume your calculation, only to find that the residuals begin to increase. This often results from increasing the underrelaxation factors too much. A cautious approach is to save a data file before making any changes to the underrelaxation factors, and to give the solution algorithm a few iterations to adjust to the new parameters. Typically, an increase in the underrelaxation 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 underrelaxed from iteration to iteration. Also, if the enthalpy equation is solved directly instead of the temperature equation (i.e., for nonpremixed combustion calculations), the update of temperature based on enthalpy will be underrelaxed. To see the default underrelaxation factors, you can click the Default button in the Solution Controls panel.
For most flows, the default underrelaxation factors do not usually require modification. If unstable or divergent behavior is observed, however, you need to reduce the underrelaxation factors for pressure, momentum, , 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 underrelaxation for SIMPLEC.) In problems where density is strongly coupled with temperature, as in veryhighRayleighnumber natural or mixedconvection flows, it is wise to also underrelax the temperature equation and/or density (i.e., use an underrelaxation 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 underrelaxation factor for temperature can be set to 1.0.
For other scalar equations (e.g., swirl, species, mixture fraction and variance) the default underrelaxation 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 underrelaxation factors in the Solution Controls panel (Figure 25.8.1).
Solve Controls Solution...
You can set the underrelaxation factor for each equation in the field next to its name under UnderRelaxation Factors.

If you are using the pressurebased solver, all equations will have an associated underrelaxation factor (Section
25.4.4). If you are using the densitybased solver, only those equations that are solved sequentially (see Section
25.1.2) will have underrelaxation factors.

If you change underrelaxation factors, but you then want to return to FLUENT's default settings, you can click the Default button. FLUENT will change the factors to the default values and the Default button will become the Reset button. To get your settings back again, you can click the Reset button.
Note that with optimal settings, the convergence of the coupled pressurevelocity 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.