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-   -   root mean square velocity (http://www.cfd-online.com/Forums/cfx/91588-root-mean-square-velocity.html)

Far August 16, 2011 07:25

root mean square velocity
 
I am simulating the flat plate with finite thickness. I am using fluent 12.1 and running URANS simulation. I am interested in the wake behaviour of different trailing edge shapes.

During this process I would like to plot the temporal rate of change of fluctuation velocity (root mean square) that is

\frac{\partial \ V_{rms}}{\partial t}

Far August 16, 2011 07:26

err : software is CFX 12.1

mvoss August 17, 2011 08:24

switch the post to the fluent branch...?

Far August 17, 2011 10:15

using both codes

ghorrocks August 17, 2011 18:38

You can do this in CFX using the transient statistics. Look on the output tab in CFX-Pre.

Mina_Shahi February 13, 2013 09:24

Quote:

Originally Posted by ghorrocks (Post 320536)
You can do this in CFX using the transient statistics. Look on the output tab in CFX-Pre.

Hi Glenn

I need to plot rms value of the stream wise velocity fluctuation in my fluid domain(√(u' u' )) .
since before running the transient case i specify velocity in the transinet statistics. so in my variables i have velocity correlation uu.Trnavg.
is that sqrt(velocity correlation uu) representing (√(u' u' )) ?

Mina_Shahi February 13, 2013 09:28

phase averaging data in CFX
 
another question:

when you specify velocity or other variables in the transinet statistics. in variables you have variable.Trnavg.
which means data has been average over the whole simulated time. but if you want to do phase averaging or make data averaged over just few periods of time is that possible?

ghorrocks February 13, 2013 18:12

If you want only partial time averaging or phase averaging you will have to export it and do it yourself.

Mina_Shahi February 14, 2013 06:53

Quote:

Originally Posted by ghorrocks (Post 407693)
If you want only partial time averaging or phase averaging you will have to export it and do it yourself.

Thank you Glenn!

what about "velocity correlation uu" is that representing (u' u' )?

or is sqrt(velocity correlation uu. Trnavg) representing (√(u' u' ))? i want to plot the rms value of velocity fluctuation, is that the right way?

ghorrocks February 14, 2013 17:12

If you are using a turbulence model then you can get u'u' from the turbulence intensity or the Reynolds Stresses (if a RSM). You can also do the Trnavg functions on these variables.

Be careful with averaging of these functions. Note that sqrt(transient average(u'u')) does NOT equal transient average(sqrt(u'u')).

Mina_Shahi February 17, 2013 06:40

Quote:

Originally Posted by ghorrocks (Post 407892)
If you are using a turbulence model then you can get u'u' from the turbulence intensity or the Reynolds Stresses (if a RSM). You can also do the Trnavg functions on these variables..


thank you for your answer, i am a bit confused with that!
in the Transient statistics there are different options: 1/ Arithmetic Average
2/ Root Mean Square and ....
In my transient simulation i used the first option Arithmetic Average and then as a variable i selected velocity ...
now as i mentioned previously i am interested in the rms axial velocity. so i am plotting the sqrt(velocity correlation uu. Trnavg) which i am not sure if it gives me rms axial velocity or not ! that is my first question !!!
i think i had to go for the Root Mean Square in the Transient statistics option, am i right?
if the transient calculation finished is that a way to get Root Mean Square values?


Quote:

Originally Posted by ghorrocks (Post 407892)
Be careful with averaging of these functions. Note that sqrt(transient average(u'u')) does NOT equal transient average(sqrt(u'u')).

which one i should look for? apparently sqrt(velocity correlation uu. Trnavg) gives sqrt(transient average(u'u'))

ghorrocks February 17, 2013 06:50

Read the documentation on the "velocity correlation uu" variable. It is not the axial velocity.


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