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UnderGroundMan November 7, 2008 14:49

what are reynolds stresses
 
Hi

When I run LES problem I get variables called reynolds stresses in post. Physically what are reynolds stresses? Thank you.

CDE November 7, 2008 21:46

Re: what are reynolds stresses
 
Reynolds stresses arise from the time averaging of the Navier-Stokes equations. Physically, they are the product of the instantaneous turbulent velocity fluctuations. You will find a detailed explanation in any textbook covering CFD.

underGroundMan November 8, 2008 11:43

Re: what are reynolds stresses
 
Hi CDE

In LES Navier-Stokes equations are not time averaged?

Regards

CDE November 9, 2008 16:32

Re: what are reynolds stresses
 
Good point! The governing equations employed for LES are obtained by filtering the time-dependent Navier-Stokes equations, which effectively filters out the eddies whose scales are smaller than the filter width and this process gives rise to subgrid-scale stresses

Paolo Lampitella November 10, 2008 04:07

Re: what are reynolds stresses
 
If you mean the fluent post, these are made by time averaging the LES solution, so they are not a part of the subgrid stress tensor, which involves space averaging.

That is, in LES, space averaging the nonlinear term gives you a subgrid stress tensor made by:

(Ui_m * Uj_m)_m - Ui_m * Uj_m = Leonard Stress Tensor

(Ui_m * (Uj - Uj_m))_m + (Uj_m * (Ui - Ui_m))_m = Cross Stress Tensor

((Ui - Ui_m) * (Uj - Uj_m))_m = Reynolds Stress Tensor

where ()_m stands for space average. Actually, in Fluent, Ui_m is just the computed solution and you don't know the fluctuating part (Ui - Ui_m) where Ui is the true, not averaged, solution, not achievable on a non-DNS grid.

What fluent does for giving you the stress reynolds is performing a time average ()_t, that is:

((Ui_m - (Ui_m)_t) * (Uj_m - (Uj_m)_t))_t = Fluent's Reynolds Stress


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