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April 4, 2013, 15:18 
How to calculate <w'T'>?

#1 
Member
Join Date: Dec 2011
Posts: 66
Rep Power: 6 
Hi guys,
I am simulating DNS channel flows using Openfoam, and I want to show contours of <w'T'>, where <> means average, w' is the zdirection velocity fluctuation, and T' is temperature fluctuation. I tried to define a new variable Q=w*T and average Q by adding the following lines into system/controlDict. But it seems to be incorrect because it will calculate <(wT)'> instead of <w'T'>. Code:
fieldAverage1 { ... fields ( Q { mean on; prime2Mean on; base time; } ... 

April 4, 2013, 16:59 

#2 
Senior Member
Lieven
Join Date: Dec 2011
Location: Mol, Belgium
Posts: 296
Rep Power: 14 
I guess the easiest way would be to do two simulations.
1. During the first run you calculate <w> and <T> 2. During the second run you read in <w> and <T>, and evaluate Q = (w<w>)*(T<T>) and take the average of Q like you are doing I'm aware that this is a bit unfavourable from computational point of view, but I don't really see an other option. If you find one however, let me know! cause this is an interesting issue. Cheers, L 

April 4, 2013, 21:23 

#3  
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Join Date: Dec 2011
Posts: 66
Rep Power: 6 
Quote:
Thanks very much for your reply! Based on your suggestion, I am thinking may be I can do it in one run. I will try the following procedures: 1. Define new variables w (U.component(2)), wMean (<w>) and TMean (<T>) in solver and read their values from meshes (an initial value is given at starttime). 2. Define new variable Q=(wwMean)*(TTMean). (that is the instant w'T' value, not averaged one) 3. Add the following average lines in system/controlDict to calculate wMean, TMean, and QMean (that is <w'T'> we want). Code:
fieldAverage1 { ..... fields ( .... w { mean on; prime2Mean on; base time; } T { mean on; prime2Mean on; base time; } Q { mean on; prime2Mean on; base time; } ..... I have added the lines mentioned above into my solver, and it is running, no error. However, I need to run a real simulation to verify if the above method produce correct results. Cheers! 

April 5, 2013, 03:00 

#4 
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Dejan Morar
Join Date: Nov 2010
Posts: 78
Rep Power: 8 
Hi ripperjack,
you just have to add a new field in your solve, wT, and then average it. You wil also need averaged values of w and T. At the end, you can modify postChannel utility to calculate <w'T'>. <w'T'> = <wT>  <w><T> Regards 

April 5, 2013, 09:13 

#5  
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Join Date: Dec 2011
Posts: 66
Rep Power: 6 
Quote:
Great! That the simplest way to do it! Thanks Morard! Regards 

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