DES Relative Length Scale in Fluent never positive, why?
 

Hello,

We are trying to do DES simulations around a bluff-body. We choose k-eps realizable as the RANS model in DES and we have appropriate mesh resolution near walls for advanced wall treatment.

We want to check which parts of the flow are handled with LES. I attached a sample contour of Relative Length Scale (DES) on a horizontal cut-plane.

The whole domain does not contain any elements with positive values of relative length scale. You see on the contour that there are very small negative values, but no positive values.

What might be the meaning of this? Should we not use relative length scale to distinguish between RANS and DES regions? I couldn't imagine that the whole domain is in RANS mode, as we have quite different results with much more visible eddies in DES simulations than in pure RANS simulations. So I assume there must be a LES region.

Thank you very much in advance for your reply. We are in a critical situation.

Additional Note:
Fluent Documentation:
Relative Length Scale (DES) (in the Turbulence... category) is defined by
Ls = Ls_rans - Ls_les (30.4-24)

where Ls_rans is an RANS-based length scale, and Ls_les is an LES-based length scale. All of the cells inside the domain in which Ls > 0 belong to the LES region, and all of the cells inside the domain in which Ls < 0 belong to the RANS region.

Hello again,

Nobody having any opinion or any experience with distinguishing between LES and RANS activated regions in DES?

I would really appreciate your suggestions.