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

RANS: difference between R and uPrime2Mean

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

Like Tree4Likes
  • 4 Post By Bernhard

Reply
 
LinkBack Thread Tools Display Modes
Old   October 10, 2013, 10:52
Default RANS: difference between R and uPrime2Mean
  #1
Senior Member
 
Samuele Z
Join Date: Oct 2009
Location: Mozzate - Co - Italy
Posts: 490
Rep Power: 9
samiam1000 is on a distinguished road
Dear All,

I am running a RANS (simpleFoam) in a channel flow.

As far as the post-processing is concerned, I am trying to figure out the difference between R and uPrime2Mean.

Can anyone help?

Thanks,
Samuele
samiam1000 is offline   Reply With Quote

Old   October 25, 2013, 05:03
Default
  #2
Senior Member
 
Bobi
Join Date: Oct 2012
Posts: 286
Rep Power: 5
babakflame is on a distinguished road
Hi Samuele
I think uprime2mean is the rms (root mean square) data that is also available from experiments, but what is R?
If you explain it maybe I could help you.
However I got another doubt.

Is there any way to calculate rms values (velocity, mixture fraction, etc) in RAS simulations in openFoam?

I posted this question long times ago, unfortunately with no reply.

I would be very glad if get any hint on this issue.

Regards
Bobi
babakflame is offline   Reply With Quote

Old   November 2, 2013, 05:13
Default
  #3
Senior Member
 
Bobi
Join Date: Oct 2012
Posts: 286
Rep Power: 5
babakflame is on a distinguished road
Greetings to all

A hint for guys who want to have rms values in RAS simulations:

Your flow solver must be unsteady i.e. pimple or piso. Since this data processing needs to move in real times.

Regards
Bobi
babakflame is offline   Reply With Quote

Old   November 4, 2013, 03:13
Default
  #4
Senior Member
 
Bernhard
Join Date: Sep 2009
Location: Delft
Posts: 790
Rep Power: 12
Bernhard is on a distinguished road
Quote:
Originally Posted by samiam1000 View Post
I am trying to figure out the difference between R and uPrime2Mean.
R is the modelled Reynolds stress tensor, which you would typically obtain with Reynold Stress Models. UPrime2Mean follows from an averaging function object and denotes < U_i U_j > - < U_i><U_j>, where U is just the velocity of your simulation. So this is basically the "resolved" part of your Reynolds stress tensor.
Bernhard is offline   Reply With Quote

Old   April 15, 2014, 13:27
Default
  #5
Senior Member
 
Join Date: Nov 2012
Location: Bavaria
Posts: 144
Rep Power: 4
aylalisa is on a distinguished road
Dear Bernhard,

is UPrime2Mean the residual-stress tensor \tau_{ij}^R that results from the filtered momentum equation??? If yes, does it include the isotropic residual stress?

I've seen from the code that postChannel utility is supposed to deliver different values like: txx, txy,tyy, txy, eps, prod, vorticity, enstrophy and helicity.
postChannel.C file lists the header files: readFields.H, calculateFields.H and collapse.H
but the content of caculateFields.H is commented out! Why is that code inactive?

It computes txx, txy etc. . Are these values square root values of statistical moments?
What's the meaning of Txx? How can this field be extracted?


Could you maybe explain your answer more detailed please

Best regards,
Aylalisa
aylalisa is offline   Reply With Quote

Old   June 10, 2014, 20:19
Default
  #6
Member
 
Fluid Dynamics
Join Date: Mar 2013
Posts: 41
Rep Power: 4
cfd.with.openfoam is on a distinguished road
Hi aylalisa,

Did you get an answer to your question?

I have simulated the Channel395 case with a refined mesh but I don't know how get the contribution of the SGS terms using SGS TKE (as I am using the one eq model) - so that I can compare with the DNS data.

Thanks
cfd.with.openfoam is offline   Reply With Quote

Old   June 11, 2014, 15:07
Default
  #7
Senior Member
 
Join Date: Nov 2012
Location: Bavaria
Posts: 144
Rep Power: 4
aylalisa is on a distinguished road
Hi cfd!

I didn't get the answer.

According to my understanding (I am not sure if I am right) you can, once your system has reached the statistically stationary state, temporally average the Reynolds-Stress tensor that includes the fluctuations <u^{''}u^{''}>,<v^{''}v^{''}>,<w^{''}w^{''}>,<u^{''}v^{''}>,<u^{''}w^{''}>,<v^{''}w^{''}> with help of function objects as described in the Forum:

Quote:
fieldAverage1
{
type fieldAverage;
functionObjectLibs ( "libfieldFunctionObjects.so" );
enabled true;
outputControl outputTime;

fields
(
U
{
mean on;
prime2Mean on;
base time;
}
);
}
prime2Mean : u_{ij}^{''} (one-point, one-time correlations), since its extracted from temporally averaged velocity field U you have <u_{ij}^{''}> (for graphs for DNS?)

Or use utility postChannel. Assuming your system is in the statistically stationary state and you have defined mesh periodicity in x and z directions the fluctuations become self-similar. Then postChannel collapses data in x and z direction, the remaining dependency is the y-direction. You find this data prepared for plotting in the new folder 'graphs'.

sgs kinetic energy - see answer of Sylv

I'd like to create plots like u_{rms}^+and<uv>^+ over y and <u>^+ over y^+.
Therefore I want to collapse time averaged fields e.g. prime2Mean in x and z. postChannel collapses different fields for each time step but not time averaged fields. How can I solve that?

The other question concerns y^+. How can I compute y^+ for a cell that is not located on the wall?


Aylalisa
aylalisa is offline   Reply With Quote

Old   June 11, 2014, 23:47
Default
  #8
Member
 
Fluid Dynamics
Join Date: Mar 2013
Posts: 41
Rep Power: 4
cfd.with.openfoam is on a distinguished road
Hi Aylalisa,

Thanks for your response.

I think my message was not that clear - so please take a look at my two detailed questions on this thread.

Thank you very much again.
cfd.with.openfoam is offline   Reply With Quote

Old   June 11, 2014, 23:59
Default
  #9
Member
 
Fluid Dynamics
Join Date: Mar 2013
Posts: 41
Rep Power: 4
cfd.with.openfoam is on a distinguished road
Hi again,

By the way I am already aware of the SGS TKE message from Sylv to your question. However, I am still not clear how to get the SGS stresses from SGS TKE. Maybe it is really stupid to ask or maybe not. I certainly need to get comfortable with these stresses - first in OpenFOAM (i.e how they are calculated) and then the physical picture. I have a background in RANS and all this is a little new.

Overall, at this point my aim is to reproduce figure 5.4 (on page 168) from Eugene's thesis.

Thanks
cfd.with.openfoam is offline   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
Relation between k and UPrime2Mean etc in LES Tarak OpenFOAM 42 October 19, 2013 06:26
Where can I find the code about Uprime2Mean and Pprime2mean Sunxing OpenFOAM 4 April 16, 2013 04:23
Why Uprime2mean is minus in some grids? maysmech OpenFOAM 19 November 14, 2012 08:31
Problems sampling UPrime2Mean leonardo.morita OpenFOAM Post-Processing 5 May 10, 2012 04:36
Difference between oodles and "oodLagrangianFoam flying OpenFOAM Running, Solving & CFD 2 April 23, 2009 13:55


All times are GMT -4. The time now is 07:11.