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-   -   Filtration in a tank (http://www.cfd-online.com/Forums/flow-3d/27340-filtration-tank.html)

Fran February 2, 2009 12:32

Filtration in a tank
 
Hi all

We have now working in a case of a desalinisation tanks. These tanks filters towards the bottom surface and maintain a fixed level whereas the water come from some inlets on the top. What we have done to simulate this is to set at the bottom of the tank a BC of an static pressure equals the fixed level. I think that setting this, F3D will permit the flow to escape from the tank whenever the lever starts increasing due to the filling. Am I doing it correctly?

In this bottom I set a F=1 so the water could come again into the tank from that bottom. How can I do to avoid this behaviour?. It is like a check valve what I am trying to simulate. Can I do it setting a F=0 or will I have problems?

P.D. We do not know at all the characteristics of the filtering media so nothing we can set about it. We only the fixed level.

Thanks in advance Fran

michael barkhudarov February 3, 2009 01:25

Re: Filtration in a tank
 
Fran,

There is no boundary condition in FLOW-3D that would act like a one-way valve. But here are a couple of tips that may help. By default, a pressure BC is a stagnation-type boundary, which means it is easier for the fluid to exit than to enter at that boundary. This should help prevent the re-etrance of water. Secondly, when figuring out the pressure value at the boundary to balance the water level above, keep in mind that the boundary pressure is set in the middle of the boundary cell, that is, half a cell outside the boundary. So you need to add that half a cell to the water depth when computing the hydrostatic pressure.

I hope it helps.

Michael

Fran February 3, 2009 02:50

Re: Filtration in a tank
 
Michael, just for clarification. I do not really reach to understand why an stagnation pressure and not a static pressure. Stagnation pressure is equal as the total pressure so we are also considering the dynamic pressure if we set in the BC only the static pressure at the bottom of the tank and we say that this is the SP then we are missing the velocity. So, can we set the Stgntion P in a BC where we do not know the velocity and therefore the Dynmc P. Thanks in advance

michael barkhudarov February 9, 2009 00:21

Re: Filtration in a tank
 
Hi Francisco,

The difference in the static and stagnation pressure at the fixed-pressure BC in FLOW-3D is how the velocity flux u*du/dn (u is normal velocity, n the normal direction to the boundary) is treated when the fluid is trying to enter the domain. For the static pressure BC, du/dn=0, while for the stagnation pressure BC it is computed assuming the fluid "outside" the boundary is at rest (u=0.0). This effectively translates into setting the full pressure p+rho*u*u/2 at the boundary.

When fluid is leaving the domain at a pressure boundary, there is no difference between static and stagnation types.

You can also look up the description in the Manual; it has more details.

Michael


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