[dickes]

Hi everyone,

I am having problems implementing the boundary conditions for my openings properly.

First of all: I am simulating liquid drainage in a foam with a transient Multiphase approach. As I am not planning to resolve the interfaces between liquid and gas bubbles I am using an Euler-Euler approach in CFX. Instead I am using momentum sources for each phase to model the flow. Right now my domain is quasi 1-dimensional (1 element in x and z which are Symmetry planes, as well as 100 elements in y) and gravity is acting in the y-direction. I initiliase the domain with liquid=0.05 (continuous Fluid) and gas=0.95 (Dispersed Fluid)

My momentum Sources are added by a subdomain for the liquid Phase as follows:
1. Isotropic Loss Model with a permeability expression, which is dependent on liquid Volume Fraction
Foam permeability = (edge length)^2*6.3*10^-3
edge length = (1-liquid.Volume Fraction)^(-1/3)*2^(-5/3)*3^(1/6)*pi^(1/3)*d gas

2. Momentum source for the capillarity:
capillary y = -surface tension/(2*0.33*diameter_gas/2)*liquid.Volume Fraction^(-3/2)*(liquid.Volume Fraction.Gradient Y)*liquid.Volume Fraction^(2)

On top of the domain I have a pressure Opening with an Opening Pressure = 0Pa; liquid.Volume fraction = 0; gas.Volume Fraction = 1

Below I also have an opening where the liquid is able to drain through, and the boundary conditions are given by liquid.Volume Fraction= 0.05; gas Volume Fraction = 0.95 and I enter the velocity in the form of

Foam permeability *(liquid.density*gravity y+capillary y)/liquid.viscosity*liquid.Volume Fraction

Which must be the velocity according to my momentum sources.

I expect the liquid to drain from the top but stay at the liquid.Volume Fraction of 0.05 at the lower boundary, which I defined as a Boundary Condition. Through the increase of the gradient at the boundary my capillary term should be growing and reduce the velocity for the outlet, untill an equilibrium is reached. However, my liquid Volume Fraction distribution will not reach an expected equilibrium. The Volume Fraction decrease below the given values set by my boundary condition and if I let the simulation run long enough, liquid will completely drain out of the Domain.

Thanks in advance for all helpful tips!