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12.22.4 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:

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  9.5 as well.



Under-Relaxation of the Reynolds Stresses


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

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

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.



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 Solution Controls panel.

Solve $\rightarrow$ Controls $\rightarrow$ Solution...



Residual Reporting for the RSM


When you use the RSM for turbulence, 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.


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