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January 29, 2011, 15:54 
Easy way to take surface average at different planes?

#1 
Member
Charlie
Join Date: Dec 2010
Location: 415 Kinross Dr. Newark, DE 19711
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Hi Foamers:
Now I'm working on channelFoam to reproduce the loglaw and turbulence intensity. I just finished my first try using dynSmagorinsky. However, I've got some problems about the postProcessing. 1. in the log file, I found that ************************************************** ************************************************** * Time = 10 ............ Uncorrected Ubar = 0.1335 pressure gradient = 0.000133009 ExecutionTime = 172139 s ClockTime = 345743 s Calculating averages ************************************************** ************************************************** ** Is the 'pressure gradient' normalized by 'rho' already? I've seem some threads about it and got confused. Basically I need to use it to calculate the wall shear velocity, so it's very important to understand that. 2. Although in the 'controlDict' file, we have fieldAverage, but It seems that it is just a time average in the UMean, Pmean files, so I have 2 questions on that: a.> iis it averaged from t=0? even if it's not steady state? b>. Is there any existing utility that I can use to take a surface average of U and P? BTW, I may need to take surface average at several planes, so maybe sampling is not such a good choice for me. Any suggestions? Thank you, foamers! Zhen 

January 31, 2011, 14:52 

#2  
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Steven van Haren
Join Date: Aug 2010
Location: The Netherlands
Posts: 149
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Quote:
Quote:
b) If you play around with the 'sample' utility you can do quite a lot. See your source code for an example of the sampleDict file. 

January 31, 2011, 23:35 
thank you!

#3 
Member
Charlie
Join Date: Dec 2010
Location: 415 Kinross Dr. Newark, DE 19711
Posts: 72
Rep Power: 5 
Hi Steven,
Thank you very much, it's really helpful. I've looked into the source file for the field average, and yes, the average is taken from t=0 if you didn't specify the starting point of averaging. Also the postChannel utility turns out to be a good choice for analyzing the channel flow. Thank you! Zhen 

October 19, 2011, 17:46 
postChannel

#4 
New Member
Join Date: Mar 2011
Posts: 12
Rep Power: 5 
Hi Charlie,
I'm trying to run the channel395 tutorial. I encounter troubles at the post processing step. I added k in the fieldAverage function present in the controlDict. My questions is: I obtain for k after postChannel utility: k kMean k.xy  I assume k is the SGS turbulent kinetic energy (based on previous posts concerning post processing of LES) kMean is therefore <k> "time averaged value". k.xy would therefore be the <k> value averaged over the homogenous directions?  What is exaclty UPrime2Mean and u.xy, v.xy, w.xy?? I assume UPrime2Mean=<u_i' u_j'> and u.xy to be u_rms> Sqrt(<u'u'>)? Am I right?  Any inside would be apreciable.. How can I compare my k to k obtained by DNS?? I don't know how to account for the SGS turbulent kinetic energy. Question solved. Last edited by tonkiplis; October 25, 2011 at 02:08. 

Tags 
pressure gradient, surface average 
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