I have struggled a lot with boundary conditions for multiphase flow, particularly where the multiphase spray is impinging on the outlet patch (typically the case of an internal flow simulation of a fuel injector spray). I have finally settled on the following Boundary conditions which work!

Simulation details: I am doing a 2D axisymmetric flow simulation using a wedge BC at front/ back planes. I am using a custom solver based on homogeneous relaxation model. This is similar to cavitatingFoam solver available in OpenFOAM which uses homogeneous equilibrium model.

Mesh: For multiphase flow, it is very very important to ensure a good quality mesh, which includes the following:
1. Avoid refinement zones - At the refinement zones boundaries, I saw spurious pressure fluctuations. Therefore, avoid them at all costs.
2. Low aspect ratio cells - Ensure that the aspect ratio of cells remains low.
3. Smooth grading in cells


Boundary conditions:

I tried several combinations for pressure boundary condition but ended up getting pressure oscillations within the domain due to the two phase flow at the outlet patch. The solution I found is to use totalPressure BC for pressure at Inlet, outlet patch.

BCs combination I finally settled on:

Pressure: totalPressure at Inlet, Outlet
Velocity: pressureInletVelocity at Inlet, pressureInletOutletVelocity at Outlet
density: zeroGradient works (I use a custom BC which is similar to ZG)


Refer to the following link for useful BC combinations for internal flow:


https://www.openfoam.com/documentati...binations.html