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Old   August 16, 2011, 07:25
Default root mean square velocity
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Sijal Ahmed Memon (turboenginner@gmail.com)
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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}
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Old   August 16, 2011, 07:26
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err : software is CFX 12.1
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Old   August 17, 2011, 08:24
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switch the post to the fluent branch...?
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Old   August 17, 2011, 10:15
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using both codes
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Old   August 17, 2011, 18:38
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You can do this in CFX using the transient statistics. Look on the output tab in CFX-Pre.
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Old   February 13, 2013, 09:24
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Quote:
Originally Posted by ghorrocks View Post
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' )) ?
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Old   February 13, 2013, 09:28
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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?
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Old   February 13, 2013, 18:12
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If you want only partial time averaging or phase averaging you will have to export it and do it yourself.
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Old   February 14, 2013, 06:53
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Quote:
Originally Posted by ghorrocks View Post
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?

Last edited by Mina_Shahi; February 14, 2013 at 07:17.
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Old   February 14, 2013, 17:12
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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')).
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Old   February 17, 2013, 06:40
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Quote:
Originally Posted by ghorrocks View Post
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?


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Originally Posted by ghorrocks View Post
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'))
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Old   February 17, 2013, 06:50
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Read the documentation on the "velocity correlation uu" variable. It is not the axial velocity.
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