The possible causes for large turbulent viscosity ratio include: – Bad initial conditions for the turbulence quantities (k and e) – Improper turbulent boundary conditions – Skewed cells If the problem is not caused by bad mesh, then the onset of the phenomena can usually be avoided by: -Turn off solving turbulence equations for the first 100-200 iterations -Turn on turbulence and continue iterations If the problem occurs in the midst of the iteration process, then use the following procedure: – Stop the iteration – Turn off all equations except the turbulence equations – Increase turbulence URFs (k and e) to 1 and iterate for 20-50 iterations – Turn back all equations and reduce the turbulence URFs to 0.5-0.8 and then continue iterations – Repeat the above steps for several times For faster convergence, it might be useful to obtain an initial solution with the standard k-e model before switching to a more advanced turbulence model. Because some turbulent flows are inherently unsteady, verify if the flow has any unsteady behavior and switch to transient calculation if needed.