Why am I getting a turbulent viscosity ratio warning message in the console when running a case?

The message you are seeing is more a warning than an error. It stipulates that the ratio of turbulent viscosity over the laminar viscosity is large than 1e5. While this is not an error per-say, it is usually non-physical to have such a high ratio (note that this ratio limit is defined in Solve-> Controls -> Limits). This ratio becomes very high in region of high turbulence, where the grid is (in most cases) too coarse. In many case, this message occurs at the beginning of a simulation and will disappear as the simulation converges.

Generally speaking, the turbulent viscosity ratio warning could occur because of four primary reasons

1. If the scale of your mesh is non-physical. Sometimes you create your model in mm and you forget to set the right units in FLUENT when you read the mesh. Make sure the scale of the model is correct.

2. If the inlet boundary condition values for turbulence parameters are high. Make sure you set reasonable values of the turbulence parameters. It is recommended to use the Intensity and (turbulent viscosity ratio or length scale or hydraulic diameter) instead of k and epsilon while specifying turbulence parameters.

3. If the initial condition values of k and epsilon are non-physically high. Make sure that these values too are relatively low

4. If you start the simulation with second order schemes for k and epsilon. Make sure you start with first order and move to second order. If the warning still continues, you can use first order for epsilon as the epsilon equation is more sensitive and difficult to converge (and is mostly the culprit behind the viscosity limitation warnings)

If these four things are set correctly then the viscosity limitation warning can come because of mesh issues. If the skewness of the mesh is too high (above 0.9-0.95). Make sure that the skewness is under 0.9.

Also note that for certain types of flows, for instance atmospheric boundary layer flows, the turbulent viscosity ratio can be as high as 1e+08 or 1e+09. In such cases, the limit can be increased. Go to Solution Controls, click the Limits tab (located below the under-relaxation factors) and enter a higher value.





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