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fieldAverage meaning in steady state solution

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Old   February 17, 2021, 19:48
Question fieldAverage meaning in steady state solution
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Geovany López
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Hi everyone,

I'm currently working with a 3D cavity-based geometry using rhoSimpleFoam solver. I'd like to get the u_rms value of my simulations. I've already done my simulation in a 5M mesh and I've validated my results.

Reading another thread I know that I have to re-run my simulation and use fieldAverage function object to get UPrime2Mean and then u_rms,x=sqrt(UPrime2Meanxx) using calculator in paraview.

My question is: In a steady simulations, 'cause I used a steady solver, the answer of u_rms,x that I will get is correct? Or it is just averaging through every iteration and this is not useful, since the solution has not yet converged until the last iteration?
In case that the approach of using fieldAverage isn't correct, someone can tell me how I could get the u_rms using a steady solver?

Thanks in advance!
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Old   February 18, 2021, 15:53
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No, don't think you can use `fieldAverage` on steady simulations. There can't be any `UPrime2Mean` because there is no `UPrime` (i.e. fluctuating part) for steady simulations.

The object you are looking for is `R` for steady cases. You can use turbulenceFields function object to calculate it, and I don't think you need to rerun your completed RANS simulation. Postprocessing the simulation should be enough.
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Old   February 19, 2021, 20:39
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Thank you very much for your help.

I used the function that you mentioned and then I was able to plot the turbulence properties in Matlab. But I have another question, I hope that you could help me. How can I compute u'_rms from the Reynolds Stress Tensor?

I think that in the latter option it is assuming that the velocity fluctuations are equal to \sqrt{\frac{2}{3}(k)} and that only applies to isotropic turbulence, right? Or am I mixing basic concepts?

My goal is to plot cross-stream profiles of turbulent fluctations \frac{u'_{rms}}{U_{max}}
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fieldaverage, rms velocity, steady state rans

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