CFD Online Discussion Forums - Should to put limit on the eddy viscosity for RANS

CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)

- Main CFD Forum (https://www.cfd-online.com/Forums/main/)

- - Should to put limit on the eddy viscosity for RANS (https://www.cfd-online.com/Forums/main/254003-should-put-limit-eddy-viscosity-rans.html)

Should to put limit on the eddy viscosity for RANS

hello,

I use RANS to similate open-channel flows and density flows. The eddy viscosity is calculated by the k-epsilon model. However, some code put upper limit on the eddy viscosity which make sure that the eddy viscosity is always below 100×snu (100 times of Coefficient of kinematic viscosity, 1e-6). I want to konw why does this limitation and is there any suggestion on the limitation.

Thanks in advance :)

In commercial software such as Ansys Fluent and StarCCM the eddy viscosity ratio limit is usually set to 1e5.

Quote:

Generally one would only increase the Maximum Ratio if the simulation has a high Reynolds number and no errors seem to be present in the simulation setup. The turbulent| viscosity ratio is proportional to a turbulent Reynolds number, where the velocity scale is typically given as k^0.5 and the length scale as (k^1.5)/epsilon (for a model that is based on the eddy viscosity concept). If you expect high turbulent Reynolds numbers (which occur when the mean flow Reynolds number is high), you could try increasing the Maximum Ratio limit by one or two orders of magnitude.

If you do not expect high turbulent Reynolds numbers, it is likely that another problem is present in your simulation, such as an incorrect boundary condition, or a mesh that has not been scaled correctly.

Quote Reference: https://community.sw.siemens.com/s/a...-output-window

Thanks, but when the limit is 1e5, the computatuion is more likeky to divergence in my case......

Quote:

Originally Posted by G. matton (Post 863175)

Thanks, but when the limit is 1e5, the computatuion is more likeky to divergence in my case......

I think the quote in my first reply provides some hints:

Quote:

If you do not expect high turbulent Reynolds numbers, it is likely that another problem is present in your simulation, such as an incorrect boundary condition, or a mesh that has not been scaled correctly.