CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

what are reynolds stresses

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   November 7, 2008, 14:49
Default 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.
  Reply With Quote

Old   November 7, 2008, 21:46
Default 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.
  Reply With Quote

Old   November 8, 2008, 11:43
Default Re: what are reynolds stresses
  #3
underGroundMan
Guest
 
Posts: n/a
Hi CDE

In LES Navier-Stokes equations are not time averaged?

Regards
  Reply With Quote

Old   November 9, 2008, 16:32
Default 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
  Reply With Quote

Old   November 10, 2008, 04:07
Default 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
  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
what are reynolds stresses UnderGroundMan CFX 2 November 10, 2008 13:31
what are reynolds stresses UnderGroundMan Main CFD Forum 0 November 7, 2008 14:49
Reynolds stresses and second law JF Main CFD Forum 6 June 2, 2005 22:57
Reynolds stresses LES dieter FLUENT 0 October 18, 2004 04:35
Reynolds stresses Darrell Anthony Egarr FLUENT 2 January 16, 2003 09:48


All times are GMT -4. The time now is 16:52.