12.16.4 RSM-Specific Solution Strategies

Using the RSM creates a high degree of coupling between the momentum equations and the turbulent stresses in the flow, and thus the calculation can be more prone to stability and convergence difficulties than with the $k$- $\epsilon$ models. When you use the RSM, therefore, you may need to adopt special solution strategies in order to obtain a converged solution. The following strategies are generally recommended:

• Begin the calculations using the standard $k$- $\epsilon$ model. Turn on the RSM and use the $k$- $\epsilon$ solution data as a starting point for the RSM calculation.

• Use low under-relaxation factors (0.2 to 0.3) and (for the density-based solvers) a low Courant number for highly swirling flows or highly complex flows. In these cases, you may need to reduce the under-relaxation factors both for the velocities and for all of the stresses.

Instructions for setting these solution parameters are provided below. If you are applying the RSM to prediction of a highly swirling flow, you will want to consider the solution strategies discussed in Section  9.3 as well.

Under-Relaxation of the Reynolds Stresses

ANSYS FLUENT applies under-relaxation to the Reynolds stresses. You can set under-relaxation factors using the Solution Controls task page.

Solution Controls

The default settings of 0.5 are recommended for most cases. You may be able to increase these settings and speed up the convergence when the RSM solution begins to converge.

In some situations, when poor convergence is observed one might facilitate the convergence rates by modifying some of the under-relaxation values whilst leaving the others unchanged. This might be a more successful approach than the simple scaling of all under-relaxation values.

Disabling Calculation Updates of the Reynolds Stresses

In some instances, you may wish to let the current Reynolds stress field remain fixed, skipping the solution of the Reynolds transport equations while solving the other transport equations. You can activate/deactivate all Reynolds stress equations in the Equations dialog box, accessed from the Solution Controls task page.

Solution Controls Equations...

Residual Reporting for the RSM

When you use the RSM for turbulence, ANSYS FLUENT reports the equation residuals for the individual Reynolds stress transport equations. You can apply the usual convergence criteria to the Reynolds stress residuals: normalized residuals in the range of $10^{-3}$ usually indicate a practically-converged solution. However, you may need to apply tighter convergence criteria (below $10^{-4}$) to ensure full convergence.