# How to find the velocity gradient and variance of velocity fluctuations

 Register Blogs Members List Search Today's Posts Mark Forums Read

 March 22, 2016, 03:51 How to find the velocity gradient and variance of velocity fluctuations #1 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 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. in PHOENICS without manually calculating it through the Boussinesq approximation i.e. =-2*vt*(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 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.*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.)

 Tags cham, fluctuation velocity, phoenics, velocity gradients