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How to find the velocity gradient and variance of velocity fluctuations

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Old   March 22, 2016, 04:51
Post How to find the velocity gradient and variance of velocity fluctuations
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Sharkito
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Dear all,

I am interested in finding the U velocity gradient in the x direction (dU/dx) and the variance of the velocity fluctuation in <u'^2> at a cell in PHOENICS. How do you define these variables? Is there an In-form command or other to set these variables?

Furhtermore, is there a way to calculate the variance of the velocity fluctuation e.g. <u'^2> in PHOENICS without manually calculating it through the Boussinesq approximation i.e.

<u'^2>=-2*vt*(dU/dx)+(2/3)*k

Thank you in advance

Last edited by Sharkito; March 30, 2016 at 03:42.
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Old   March 30, 2016, 03:41
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Below you will find the answer to the question above taken from a communication with the excellent support of CHAM. In blue, you will find a extract of this communication.

If you store 3d variables named DUDX, DVDY and DWDZ in the VR Editor, then PHOENICS will compute these gradients automatically.

For the standard k-e model, the inform commands that derive the turbulent normal stresses from the field values using the Boussinesq stress-strain relationship are:


************************************************** **********
Echo save-block settings for Group 7
save7begin
(stored of U1SQ is -2.*ENUT*DUDX+2.*KE/3. with IMAT<100!ZSLFIN)
(stored of V1SQ is -2.*ENUT*DVDY+2.*KE/3. with IMAT<100!ZSLFIN)
(stored of W1SQ is -2.*ENUT*DWDZ+2.*KE/3. with IMAT<100!ZSLFIN)
save7end
************************************************** **********


In addition to storing these variables as stated above in Group 7 one needs to include the following commands in Group 7:
  • You will need to "NAME" the variables this is done by inserting the command e.g. NAME(146)=DUDX NAME(146)=DVDY etc.
  • Then one needs to "STORE" the variables this is done by STORE(DUDX,DVDY, etc.)
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cham, fluctuation velocity, phoenics, velocity gradients

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