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-   -   VOF Outlet boundary condition in cfd - ace (http://www.cfd-online.com/Forums/main/12705-vof-outlet-boundary-condition-cfd-ace.html)

JM December 15, 2006 09:07

VOF Outlet boundary condition in cfd - ace
 
Hello,

I am trying to simulate a tank emptying process filled with benzin. In reality the tank is emptied through gravity. I switched on VOF module in CFD - ace to track this free surface. Initially I just used a pressure outlet condition with VOF parameter as Fluid2 = 1 (there are three possibilities in this section. Fluid1 =1, Fluid2=1 or mixture where I have to define my own fractions). The process is like this, the tank is emptied and when the liquid level is less, a vortex is formed just above the outlet point which causes the suction of air as well with it (Fluid2 = 1, doesnt hold true anymore in this situation). The process is turbulent (about 850l/min in a pipe of 80 mm).

1) When I just had outlet (no inlet as i thought nothing is coming in anyway), with pressure outlet BC. there was no flow at all.

2) Then I created an inlet which was in my case, the top wall of the tank (assuming when the tank is emptied, the air occupies the emptied volume). the outlet was still pressure BC (bit strange that both the inlet and the outlet BCs as pressure BCs, but I dont find any other reasonable BC condition as the outlet flow is purely gravity based and I cannot calculate the flow rate or influence the flow rate in anyway). Even in this case there was no flow at all.

3) I gave a pressure inlet BC, and velocity outlet BC (i defined a velocity myself, though this is not physical as i have no influence over the flow rate), in order to see atleast the flow. I could see the flow, track the surface going down with the time (as the liquid is being taken out through gravity). then the vortex (not clear whether this vortex was rotating like a whirlpool which we can see in our house bathtubs which is the reality). but the code abnormally stopped saying the fluid2 =1 in the VOF section of the outlet BC has not met. which is true as after some time when the liquid level is too low, the vortex induces the suction of air in the outlet and at the outlet it is not pure Fluid2 anymore. But it is the reality and how can I capture this physics? this is what I want to model as this very air in the outlet is my problem and I have to find a way to get rid of this bubbles before they come to the liquid measuring meter.

Any idea what can be played with the BCs here. All option have been more or less tried.

No Inlet --- Pressure outlet Pressure Inlet -- Pressure outlet Pressure Inlet -- Velocity outlet (flow seen, but problems when gas bubbles appear)

Haven't tried yet

Velocity inlet -- Pressure outlet (in my opinion not the reality, but maybe there is a way to manipulate something to come close to reality, experts comments?)

Gravity is switched on and I cannot switch on the two phase module in this program together with VOF.

Thanks for your assistance.


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