(porous)interFoam: Pressure in trapped air
 

Hi Foamers,

I'm trying to simulate the filling of a cavity, which is a porous zone. This cavity is initially filled with air. In the middle of the cavity, there is an obstacle, that creates an air trap.
Now, as I defined a pressure of 0 Pa on the outlet, which persists in the air filled zone, I would expect the trap to be filled up with liquid. At least, this is what Pam-RTM simulates and what happens in experiment.

But, after the air is trapped, its pressure increases suddenly and stops the flow from advancing.

Why does that happen?

I attached a sketch and my case (to be run blockMesh and porousInterFoam).

Thank you, Kind Regards

Peter

I've got an idea which I'd like to discuss.
Is it possible that the following happens in my problem?:

The alpha value is not EXACTLY zero in air filled regions.
The mixture rule in VOF-method now calculates the cell viscosity from air and liquid viscosities.
In the PISO-loop, this slightly raised viscosity leeds to a higher flow resistance which in turn raise the pressure in the trapped air zone.

This would explain why, when I lower the air viscosity to something ridiculously small, the air trap is nearly completely filled.

Best regards,
Peter

Hi,

I don't know which version of OpenFOAM you may be using, but you may also need to reformulate the alpha equation, as by default it does not take porosity into account. Check this post:

http://www.cfd-online.com/Forums/ope...tml#post431805

Regarding your problem, you may want to test the influence of cAlpha in preventing the diffusion of your interface.

Best,

Pablo

Hi Pablo,

thank you very much for your reply.

I'm using OpenFoam version 2.3.0

To take porosity into account, I use to multiply the d value in the porosityProperties with the porosity ( d = Porosity/Permeability ).
I know this is not the clean solution, but (I hope) for incompressible, laminar and creeping flows it's ok. At least my numerical results are in accordance to analytical calculation with Darcy's law.

About cAlpha: I'll test this, thank you for the advice!

Do you think it's possible to tell the solver not to calculate any pressure in the non-filled area, but to take the outlet pressure in those cells?
And if yes: Which piece of code do I have to change: pEqn.H or createFields.H?
(createFields.H is only called right at the beginning for the calculation, not in every time step, isn't it?)

Peter

Hello John Degenkolb,
First of all I know nothing of porous material. What I am wondering if why the air will leave the cavity. If this was a tube then the pressure will increase (P*V=constant) until the P is equal to the height of the water column on the outside P=rho * g* h. But maybe this is different with porosity. To completely fill the tube you need a leak at the top of the tube so the air can escape.

Hope this helps
Wouter

Hi Wouter,

I expected something like this to happen as well... Until I did my experiments :eek:

You are right with p*V=constant, but p=0 !
Now one could saw: In that case it will be filled in zero time. Well, no, as the porous media (and to a small extent the viscous forces) slow down the fluid.

I know Openfoam only works with relative pressures. But how to teach it about vacuum?

Hello John Degenkolb,

Oke, there is no air (P=0) so do not put air in, it is just a one phase system so you do not need interFoam, but maybe a dynamic mesh system

hope this helps
Wouter

I never tried dynamic meshing, but

• remeshing every time step is sad to be very time consuming and
• for complex geometries to be filled very hard to do, I think
• the commercial software is working with some kind of VOF methods as well

Thanks for your hints anyway!

hello John Degenkolb,

The Interfoam solver can be used, I think, because there are two phases: liquid and liquid vapour. Could the filling of the pores be due to condensation of the liquid vapour, because pressure increase supersaturated?

hope this gives you another thing to think about, nice problem

Wouter