Setting Boundary Conditions

Volume Fraction is specified at velocity or mass flow inlets, or pressure outlets, but not at walls or outlet boundaries. The volume fraction determines how much of that phase enters the computational domain.

The phase permeability is specified at wall and outlet boundaries. You can define a boundary as being permeable or impermeable for a particular phase. The permeable wall phase condition allows the selected phase to permeate though the wall, thus the wall behaves like an outlet for that phase. Other phases in the domain still treat the boundary as a wall. This phase condition can be used to model the deposition of a phase on a surface, or to model a gas leaving a liquid at a free surface (degassing), for example. At a phase permeable wall, any component of phase velocity parallel to the wall is treated according to the wall boundary conditions. For example, zero for a non-slip wall in a stationary geometry case.

An example where boundary permeability settings would be advantageous is modeling a bubble column containing air and water. In this case, you could specify a degassing boundary at the outlet, which would allow air to exit the fluid domain but not water. You can use the Phase Impermeable option for the water phase at the outlet boundary, so the mass of water within the column is conserved, while allowing the air bubbles to escape. Simcenter STAR-CCM+ balances the mass flow of air at the inlet and outlet of the domain and then computes the volume fractions of each phase throughout the domain according to continuity.

An example involving a permeable wall phase condition is a case that involves droplet deposition on a wall. The permeable wall approach assumes that when droplets impinge on a surface they stick there and hence are removed from the flow field. The permeable wall phase condition says that the phase leaves the flow domain at the rate that it approaches the surface. It is also assumed that the deposition layer is sufficiently thin so as not to affect the flow field. Where not all the droplets stick to the wall, the wall permeability factor could be adjusted to limit the mass flux that is removed at the wall.

The steps in this procedure are intended to follow on from Step 6 in Modeling Eulerian Multiphase Flow.

To specify the boundary conditions and values for each phase: