LES grid mesh
I'd like to make u some questions related to LES and somehow also to DES since this turbulence model is a combination of former and RANS. The point is that I read famous paper of R. Spalart about DES, and it says that the model switch from one to the other depending on which lenght is smaller. Something like:
/d = min (d, C_des·Delta_des)
in such a way when the distance to the nearest wall (d) is smaller than C_des·Delta_des (i.e the grid scale) RANS is applied and far away from the wall (d is bigger than latter) LES is applied.
I don't know a lot about fluid mechanics, but it seems that LES mode is used when large eddies appear. So if detachment is observed, doesnt it appear next to the wall and therefore LES should be applied?
Second, it seems also that LES needs very fine mesh in order to capture the eddies. My question is...if LES is used to catch the large eddies why is need to do such a fine mesh? I cannot understand why is necessary a finer mesh than RANS one...
Any suggestion and opinion is welcome, so please i'd appreciate any idea.
Thanks in advance,
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