root mean square velocity

Far · August 16, 2011, 07:25

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, 08:24

Quote:

Originally Posted by ghorrocks

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, 08:28

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, 17:12

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, 05:53

Quote:

Originally Posted by ghorrocks

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, 16:12

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, 05:40

Quote:

Originally Posted by ghorrocks

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

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, 05:50

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

ansari.nazanin · June 1, 2016, 08:58

Hello, i want also plot the rms values of the velocity and i am really confused reading this thread:
-i don't find the options you named under transient simulation in Output tab by ANSYS CFX-Pre
-Is it possible to plot these fluctuations using SST model?
- I am also doing the simulations with RSM models, how is that possiblr to plot?
- I do not find the variables with Atrnave function! Could you please help me?

August 16, 2011, 07:25	root mean square velocity	#1
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	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 $\frac{\partial \ V_{rms}}{\partial t}$

February 13, 2013, 08:28	phase averaging data in CFX	#7
Mina_Shahi Member Mina Join Date: Apr 2011 Posts: 88 Rep Power: 15	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?

June 1, 2016, 08:58	Any suggestions on how to do it?	#13
ansari.nazanin New Member Nazanin Ansari Join Date: Sep 2013 Posts: 12 Rep Power: 12	Hello, i want also plot the rms values of the velocity and i am really confused reading this thread: -i don't find the options you named under transient simulation in Output tab by ANSYS CFX-Pre -Is it possible to plot these fluctuations using SST model? - I am also doing the simulations with RSM models, how is that possiblr to plot? - I do not find the variables with Atrnave function! Could you please help me?

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August 16, 2011, 07:26		#2
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	err : software is CFX 12.1

August 17, 2011, 08:24		#3
mvoss Senior Member Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 449 Rep Power: 20	switch the post to the fluent branch...?

August 17, 2011, 10:15		#4
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	using both codes

August 17, 2011, 18:38		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	You can do this in CFX using the transient statistics. Look on the output tab in CFX-Pre.

February 13, 2013, 17:12		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If you want only partial time averaging or phase averaging you will have to export it and do it yourself.

February 14, 2013, 16:12		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	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')).

February 17, 2013, 05:50		#12
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Read the documentation on the "velocity correlation uu" variable. It is not the axial velocity.