How to find the velocity gradient and variance of velocity fluctuations

Sharkito · March 22, 2016, 03:51

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

Sharkito · March 30, 2016, 03:41

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.)

March 22, 2016, 03:51	How to find the velocity gradient and variance of velocity fluctuations	#1
Sharkito New Member Sharkito Join Date: Mar 2016 Posts: 2 Rep Power: 0	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>=-2vt(dU/dx)+(2/3)k Thank you in advance Last edited by Sharkito; March 30, 2016 at 03:42.*

March 30, 2016, 03:41		#2
Sharkito New Member Sharkito Join Date: Mar 2016 Posts: 2 Rep Power: 0	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.ENUTDUDX+2.KE/3. with IMAT<100!ZSLFIN) (stored of V1SQ is -2.ENUTDVDY+2.KE/3. with IMAT<100!ZSLFIN) (stored of W1SQ is -2.ENUTDWDZ+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.)