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Turbulent Heat Flux <u'T'>

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Old   June 5, 2013, 08:27
Default Turbulent Heat Flux <u'T'>
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Hello Foamers... I made a fairly simple modification to my solver to enable the computation of uT. Given that I am interested in the time-averaged values of this variable, I added it to the controlDict file to allow for this. Now, I understand what mean of uT is, just <uT>. However, for the prime2Mean, what would that be for uT? Is it <u'u'T'T'> ? And if so, how do I get the turbulent heat flux, <u'T'> from <u'u'T'T'>. I know there is another way using <u'T'>=<uT>-<u><T>, which is an approximation. But how can I compute the <u'T'> directly in OpenFOAM?
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Old   June 10, 2013, 06:49
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Dear deji
I used the same method as yours i.e. adding the uT variable in my solver and activating the averaging function for <uT> in controlDict of LES and then computing <u'T'> using <u'T'>=<uT>-<u><T> .
I don't know the physical meaning of prime2Mean (Variance) for u'T' and the relationship between them. If you find something, please post it in here so that everybody can use it
Thanks
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Old   June 10, 2013, 11:57
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Thanks for the response. I've actually solved the problem; the definition of <u'T'> that I described to be an "approximation" actually is not one if the averaging time is "long".
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Old   June 10, 2013, 14:24
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Quote:
Originally Posted by deji View Post
Thanks for the response. I've actually solved the problem; the definition of <u'T'> that I described to be an "approximation" actually is not one if the averaging time is "long".
What do you mean by Long?
I will consider the convergence time as the time which the flow sweeps my whole domain 3 times from inlet to outlet. (for the cases with inflow, NOT cyclic B.C.). (Although in my experience 2 Times would be sufficient and the results does not change after that)
After that, I will turn on the averaging function in LES to run. I will consider the averaging time the same as convergence time i.e. the time which the flow sweeps my whole domain 3 times from inlet to outlet. after that I will postprocess my results.
Is it the same approach that you usually use?
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Old   June 10, 2013, 16:13
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Typically, in forced convection turbulent boundary flow, to attain statistical convergence of 2nd order statistics (u'u',u'T','u'v', etc), it is "recommended" the flow sweep the computational domain 15-20 times. Some refer to this a flow-through times. However, since my research pertains to buoyancy affected wall bounded turbulent flow, I have discovered the flow needs roughly 30-40 flow through times to acquire statistical convergence of some 2nd order statistics. Thus, it greatly depends upon the kind of turbulent flow you are computing in order to properly "ascertain" how much flow-through times is required to achieve 2nd order convergence. Remember to always compare the 2nd order quantities as opposed to the 1st order quantities to determine this!
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Old   June 10, 2013, 17:44
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Quote:
Originally Posted by deji View Post
Hello Foamers... I made a fairly simple modification to my solver to enable the computation of uT. Given that I am interested in the time-averaged values of this variable, I added it to the controlDict file to allow for this. Now, I understand what mean of uT is, just <uT>. However, for the prime2Mean, what would that be for uT? Is it <u'u'T'T'> ? And if so, how do I get the turbulent heat flux, <u'T'> from <u'u'T'T'>. I know there is another way using <u'T'>=<uT>-<u><T>, which is an approximation. But how can I compute the <u'T'> directly in OpenFOAM?
Hi deli,

You can calculate <u'T'> directly if you check the source code how OF calculate <u'u'>, <u'v'>, etc. (i.e. UPrime2Mean). If you are still interested in it, I can find the code for you.

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Old   August 20, 2013, 09:59
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Hi ripperjack,

it would be very nice, if you could provide the source code to calulate the turbulent fluxes.

Tobi
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Old   August 20, 2013, 10:23
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It is actually fairly easy to compute the turbulent heat fluxes. A minor modification to the source code to calculate uT is all that's need. With the variable uTmean available, <u'T'> can be computed in a post-processing step using <u'T'>=<uT>-<u><T>, assuming the averaging time is "long enough".

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Old   August 20, 2013, 11:57
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Hi Deji,

is there also a way to calculate the fluxes at runtime?

Cheers
Tobi
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Old   August 20, 2013, 12:15
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I do believe there is a way to compute the fluxes at runtime. You'll have to modify the code to enable the calculation of <uT> ,<u> and <T> at runtime, instead of using the fieldAverage function object for this. I think it ought to be straightforward.
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Old   November 13, 2013, 14:26
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is there anyone who can provide the modification of the solver for computing uT?
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Old   November 13, 2013, 14:32
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Quote:
Originally Posted by Gosi View Post
is there anyone who can provide the modification of the solver for computing uT?
See the attached file. It is the solver I am using, hope that helps!

BTW: you can get reference at this link:

Why Uprime2mean is minus in some grids?

Regards,
Attached Files
File Type: h calculatePostVariables.H (1.4 KB, 38 views)
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Old   November 13, 2013, 14:40
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is there anyone who can provide the modification of the solver for computing uT?

It is actually a rather simple modification that is required to the code being utilized for the computation of the turbulent heat flux. You ought to compute uT in your flow solver and utilized it to calculate <u'T'> as a post-processing step.
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Old   January 23, 2014, 07:02
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Quote:
Originally Posted by cfdonline2mohsen View Post
Dear deji
I used the same method as yours i.e. adding the uT variable in my solver and activating the averaging function for <uT> in controlDict of LES and then computing <u'T'> using <u'T'>=<uT>-<u><T> .
I don't know the physical meaning of prime2Mean (Variance) for u'T' and the relationship between them. If you find something, please post it in here so that everybody can use it
Thanks
Hi, Mohsen!

I want to calculate <u'T'> too. I'm not so sure how to add uT in my solver. Could you please post your code here? Just the "adding uT" part would be fine. It would be very helpful for me and others who try to do the same thing.

Regards,
Peter
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