# what are reynolds stresses

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 November 7, 2008, 14:49 what are reynolds stresses #1 UnderGroundMan Guest   Posts: n/a Hi When I run LES problem I get variables called reynolds stresses in post. Physically what are reynolds stresses? Thank you. jzc likes this.

 November 7, 2008, 21:46 Re: what are reynolds stresses #2 CDE Guest   Posts: n/a 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.

 November 8, 2008, 11:43 Re: what are reynolds stresses #3 underGroundMan Guest   Posts: n/a Hi CDE In LES Navier-Stokes equations are not time averaged? Regards

 November 9, 2008, 16:32 Re: what are reynolds stresses #4 CDE Guest   Posts: n/a 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

 November 10, 2008, 04:07 Re: what are reynolds stresses #5 Paolo Lampitella Guest   Posts: n/a 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 jzc likes this.

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