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Fluctuating component of a scalar quantity

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Old   August 9, 2019, 13:59
Default Fluctuating component of a scalar quantity
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I'm dealing with external aerodynamics problems with dispersion of gaseous substances.
Simulations are carried out considering URANS and a Scale Adaptive approach for the turbulence model (kOmegaSSTSAS). The tracer gas is transported by means of the scalarTransport FO (i.e. as a passive scalar)

I'd like to postprocess not only the Reynolds Stress Tensor by means of the prime2Mean utility but also the turbulent diffusion flux of concentration : <u'*c'>; <v'*c'>; <w'*c'>
Related to the postProcess of these last three quantities I'm facing two problems:

- The first one is mainly conceptual and it's related to the fluctuating part of the velocity field. Since the turbulence model is the kOmegaSSTSAS, should I compute directly the fluctuations of the velocity field? Or should I consider the assumprtion made in the turbulenceIntensity FO (u'=v'=w') accounting for the fluctuating part as uPrime=sqrt(k*2/3)?
- The second is related to the code needed in order to evaluete the turbulence intensity of a scalar (c') and subsequently the turbulent diffusion fluxes (< u_i '*c' >). My intent was to somehow exploit the FO (fieldAverage.C/H and fieldAverageItem) used for prime2Mean computation but I got lost. I can't find the proper way to do that.

Thanks for reading.
Hoping to find some answers
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Old   September 9, 2019, 12:15
Default possible answer to question number 2
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I think I've menaged to do what I needed. Instead of changing fieldAverageTemplates.C with an additional variable (like, let's say, primeField), I tried to evaluate < u' c' >, < v' c' >, and < w' c' > in the following way:
mean value of U and c thanks the fieldAverage FO --> definition of the new variable U' (3 components vector) * c' as (U - UMean)*(c - cMean) --> mean value of this new variable.
I hope this is a valid approach since the fieldAverage code computes the iterative average.
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fieldaverageitem, turbulence intensity, uprime

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