12.16.3 Convergence

The suggestions below are provided to help you enhance convergence for turbulent flow calculations:

• Starting with excessively crude initial guesses for mean and turbulence quantities may cause the solution to diverge. A safe approach is to start your calculation using conservative (small) under-relaxation parameters and (for the density-based solvers) a conservative Courant number, and increase them gradually as the iterations proceed and the solution begins to settle down.

• It is also helpful for faster convergence to start with reasonable initial guesses for the $k$ and $\epsilon$ (or $k$ and $\omega$) fields. Particularly when the enhanced wall treatment is used, it is important to start with a sufficiently developed turbulence field, as recommended in Section  12.15, to avoid the need for an excessive number of iterations to develop the turbulence field.

• When you are using the RNG $k$- $\epsilon$ model, an approach that might help you achieve better convergence is to obtain a solution with the standard $k$- $\epsilon$ model before switching to the RNG model. Due to the additional non-linearities in the RNG model, lower under-relaxation factors and (for the density-based solvers) a lower Courant number might also be necessary.

Note that when you use the enhanced wall treatment, you may sometimes find during the calculation that the residual for $\epsilon$ is reported to be zero. This happens when your flow is such that Re $_y$ is less than 200 in the entire flow domain, and $\epsilon$ is obtained from the algebraic formula ( this equation in the separate Theory Guide) instead of from its transport equation.