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-   -   zero velocity gradient (http://www.cfd-online.com/Forums/phoenics/52007-zero-velocity-gradient.html)

fevzi February 10, 2006 10:31

zero velocity gradient
 
Hi, What is the correct way of setting zero velocity gradient at the outlet. Thanks.

Mick February 10, 2006 14:17

Re: zero velocity gradient
 
By default all boundaries in PHOENICS are zero flux. i.e d(phi)/dn = 0.

You should set a boundary condition that sucks the same mass out from each cell that is entering it. You could probably do this using inform, or easily with ground. But you will need to calculate\estimate the rho*vel*Area of the fluid as it enters the outlet cell for this you will need the previous cell density and velocity and cell face area. This could be done in a flash using ground.

fevzi February 11, 2006 08:53

Re: zero velocity gradient
 
Thank you Mick. Best regards

fevzi February 11, 2006 09:40

Re: zero velocity gradient
 
We are using version 3.6. In outlet attributes window, for velocities we have USER SET, IN CELL, DEDUCED options. İn order to assign zero velocity gradient at the outlet which one we should choose.And also for zero temperature gradient which option should be selected.Thank you.

Mick February 16, 2006 06:01

Re: zero velocity gradient
 
I think that you need to set the outlet BC yourself rather then use standard outlet. As I described above, this will be more like an inlet but sucking mass out of the domain.

As I said before you should be sucking out the same mass from the oulet cell that enters it. I would use ground coding, but you might try experimenting with INFORM to see what you can do. You need access to the velocity of the preceeding cell and density and cell face area .

Perhaps some ground coding.

in Q1 : Patch(exit,east,nx,nx,1,ny,1,nz,1,lstep)

coval(exit,P1,fixflu, grnd)

In ground.for in group 13, something like

IF(NPATCH(1:4).eq.'EXIT')THEN l0val=L0F(VAL) L0AE=L0F(EAST) L0DEN=L0F(DEN1) L0VEL=L0F(WEST(U1)) ! Try this to get Vel for previous cell otherwise you will have to have an Icell2 to pick up slab NX-1

do ix=1,nx

do iy=1,ny

icell =iy+(ix-1)*ny

F(L0VAL+ICELL)=-F(L0DEN1+ICELL)*F(L0AE+ICELL)*F(L0VEL1+ICELL)

enddo enddo

NOTE; you will probably need to fix a refrence pressure in the domain somewhere to anchor pressure

With a normal outlet Pressure is just set, In this case I think that if you want DT/Dn=0 the you should set ; COVAL(EXIT,TEM1,0.0,SAME) as this will ensure that if any temperature enters through the oulet because of the exit pressure value, then it will enter with the same TEMP as in the last cell.

In terms of the MENU for OUTLET type BC and TEM1 this should be set to 'IN-CELL'.

Hope this helps, has anyone else out there got any better ideas or method on this. What about trying INFORM anyone!.

Regards



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