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

Re: what are reynolds stresses
Hi CDE
In LES NavierStokes equations are not time averaged? Regards 
Re: what are reynolds stresses
Good point! The governing equations employed for LES are obtained by filtering the timedependent NavierStokes equations, which effectively filters out the eddies whose scales are smaller than the filter width and this process gives rise to subgridscale stresses

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 nonDNS 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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