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

Reynolds stresses without velocity correlations?

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 1, 2006, 17:23
Default Reynolds stresses without velocity correlations?
  #1
Phil
Guest
 
Posts: n/a
Hi all, I have a quick question about calculating the Reynolds stresses in a URANS code without using the velocity correlations directly!

Notation I'll be using:

ui Instantaneous velocity component

Mean(ui) Time arithmetic average of velocity component

u'i Flucuating component of velocity component

The Reynolds stresses are defined as:

Mean(u'iu'i) = Mean(uiui) - Mean(ui)Mean(ui)

In the closed code I am using I have easy access to the following variables:

Mean(ui)

RMS(ui)

Stddev(ui)

I dont have direct access to the velocity correlations e.g. Mean(uiui).

I can calculate the *normal* Reynolds stresses using the available variables with the following formula:

Mean(u'iu'i) = RMS(ui)RMS(ui) - Mean(ui)Mean(ui)

However, this last formula is not valid for the Reynolds *shear* stresses. For example:

Mean(u'v') = RMS(u)RMS(v) - Mean(u)Mean(v)

, is not valid and does not produce the correct values for Mean(u'v').

Does anyone know if it is even possible to calculate the Reynolds shear stresses if you only have access to Mean(ui), RMS(ui), Stddev(ui) ??

If all else fails I could create a user routine to calculate the shear velocity correlations from first principles but I'd rather use the available pre-prepared variables if that is feasible.

  Reply With Quote

Old   October 2, 2006, 05:29
Default Re: Reynolds stresses without velocity correlation
  #2
Pennysworth
Guest
 
Posts: n/a
I think if you are doing URANS then the Reynolds stresses come directly from whatever turbulence model it is you are using. It is part of the system of equations you are solveing for (i.e. it is already calculated/modleled).

> u'i Flucuating component of velocity component

I would think that this is the resolved part of the velocity fluctuation. The unresolved part is what comes from your model. Anyway, I think the following notes might be helpful:

http://www.tfd.chalmers.se/doct/comp...iles/urans.pdf

At leaset you can trust them more than me ;-)
  Reply With Quote

Reply

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
RMS velocity and reynolds stres with fluent gina FLUENT 1 May 20, 2013 05:05
Reynolds Stresses in Cylindrical Coordinates steve Main CFD Forum 0 February 7, 2009 14:38
Eh what happened to my reynolds stresses cfd toy CFX 0 July 3, 2008 16:35
Reynolds number or velocity calc hari FLUENT 0 April 21, 2008 04:29
Velocity Under-relaxation in SIMPLE type methods Matt U. Main CFD Forum 6 July 4, 2005 06:29


All times are GMT -4. The time now is 00:25.