CFD Online Logo CFD Online URL
Home > Forums > General Forums > Main CFD Forum

? fluctuating equations for homogenous shear turb.

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

LinkBack Thread Tools Search this Thread Display Modes
Old   September 19, 2002, 19:24
Default ? fluctuating equations for homogenous shear turb.
Posts: n/a
This is a question for those familiar with plane homogeneous shear turbulence. I have a reference [Rogallo (1981) 'Experiments in homogeneous turbulence'] that gives the governing equations for just the fluctuating part of the flow.

To get these, first Rogallo decomposes the velocity and pressure into mean and fluctuating components and substitutes this into the original governing equations. In the new equation, the differentiation of the mean components is with respect to an orthogonal coordinate system but the differentiation of the fluctuating components is with respect to a coordinate system that moves with the mean flow. [The point of this is to be able to apply periodic boundary conditions to the fluctuating components].

The next step he does is what I'm confused on. He states: "To separate the mean from the fluctuating parts we take the ensemble average of the Navier-Stokes equations, using the ergodic hypothesis to replace ensemble averages of spatially homogeneous terms by their volume averages"

In his equations of the mean flow, there are no Reynolds stresses. Correct me if I'm wrong, but doesn't the ergodic hypothesis state that we should get the same result for ensemble / time / space averages? I just don't understand when trying to follow his work how he gets rid of the averaged '<uv>' terms his mean equations.

I'm probably missing something obvious, but after working on it for hours I'm ready to see the 'light'.

Feel free to e-mail me at if you would wish to discuss this outside the forum. I'd be happy to explain my problem in more detail if it isn't clearly stated enough.


  Reply With Quote

Old   September 20, 2002, 07:39
Default Re: ? fluctuating equations for homogenous shear t
Posts: n/a
The volume averaged Reynolds stresses are not zero, but the gradients of the mean Reynolds stresses are zero because the flow is homogeneous. By the way, time and volume averaged Reynolds stresses are not the same in this case because the mean flow is a function of time.

  Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Guide: Writing Equations in LaTeX on the CFD Online Forums pete Site Help, Feedback & Discussions 27 May 19, 2022 03:19
Shear Stress Calculation in the flow. auburnuser FLOW-3D 11 May 12, 2018 13:49
shear stress scalar definitions? Novak Elliott CFX 0 April 6, 2003 01:45
Complicated Homogeneous Shear DNS ff_fan Main CFD Forum 0 December 9, 2002 18:23
What is the detail definition of wall shear stress zjm FLUENT 0 January 2, 2002 07:43

All times are GMT -4. The time now is 08:57.