two-phase flow - outlet BC for pressure

michielm · November 29, 2013, 11:02

Hi,
We are trying to simulate the flow of a jet of liquid falling inside a gaseous chamber and exiting at the bottom with interFoam. We can set up the simulation such that it runs, but as soon as the liquid reaches the bottom the simulation crashes.

We believe this is caused by the following issue: we have assigned a fixedValue 0 for the pressure at the outlet and zeroGradient for the velocity.

As long as there is no liquid at the outlet this is fine, but as soon as there is both liquid and gas at the outlet you will have a capillary pressure jump across the interface. This fact, combined with the boundary condition that forces the pressure at the boundary inside BOTH fluids to 0 will mess things up.

Does anyone have advice on the appropriate pressure boundary condition that will allow both liquid and gas to flow out?

LESlie · December 11, 2013, 09:09

Hi Michiel,

Are you using VOF solvers for you two phase problem? In that case the BC of alpha can also play a role. You can try inletOutlet.

I would suggest you to use totalPressure for the pressure and pressureInletOutletVelocity for the velocity.

michielm · December 12, 2013, 04:07

Hi Leslie,
Thanks for the reply. Yes, I'm using VOF (interFoam).

I currently use 'advective' as the outflow boundary condition for alpha, which seems to do exactly what I want, namely just advect alpha along with the main flow.

I have tried the totalPressure condition for the pressure and it works somewhat (there is no crash), but it doesn't really make sense that much because totalPressure sets:
$p=p_0+1/2 \rho |U|^2$ so that would suggest that $1/2 \Delta \rho |U|^2$ equals the capillary pressure jump across the interface which is typically not the case.

What is your take on that?

November 29, 2013, 11:02	two-phase flow - outlet BC for pressure	#1
michielm Member Michiel Join Date: Oct 2010 Location: Delft, Netherlands Posts: 97 Rep Power: 15	Hi, We are trying to simulate the flow of a jet of liquid falling inside a gaseous chamber and exiting at the bottom with interFoam. We can set up the simulation such that it runs, but as soon as the liquid reaches the bottom the simulation crashes. We believe this is caused by the following issue: we have assigned a fixedValue 0 for the pressure at the outlet and zeroGradient for the velocity. As long as there is no liquid at the outlet this is fine, but as soon as there is both liquid and gas at the outlet you will have a capillary pressure jump across the interface. This fact, combined with the boundary condition that forces the pressure at the boundary inside BOTH fluids to 0 will mess things up. Does anyone have advice on the appropriate pressure boundary condition that will allow both liquid and gas to flow out? Last edited by michielm; December 12, 2013 at 04:07. Reason: Added the used solver

December 11, 2013, 09:09		#2
LESlie New Member NaiXian Leslie Lu Join Date: Jun 2009 Location: France Posts: 26 Rep Power: 16	Hi Michiel, Are you using VOF solvers for you two phase problem? In that case the BC of alpha can also play a role. You can try inletOutlet. I would suggest you to use totalPressure for the pressure and pressureInletOutletVelocity for the velocity. __________________ Cheers, Leslie LU

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December 12, 2013, 04:07		#3
michielm Member Michiel Join Date: Oct 2010 Location: Delft, Netherlands Posts: 97 Rep Power: 15	Hi Leslie, Thanks for the reply. Yes, I'm using VOF (interFoam). I currently use 'advective' as the outflow boundary condition for alpha, which seems to do exactly what I want, namely just advect alpha along with the main flow. I have tried the totalPressure condition for the pressure and it works somewhat (there is no crash), but it doesn't really make sense that much because totalPressure sets: $p=p_0+1/2 \rho \|U\|^2$ so that would suggest that $1/2 \Delta \rho \|U\|^2$ equals the capillary pressure jump across the interface which is typically not the case. What is your take on that?