How do pressure outflow BC and impermeable IS combine?

MFiX
1

I am investigating the tutorial “3D PIC cyclone” which comes pre-installed with MFiX. I’ve found that the BC_3 region is a BC and IS at the same time. It has a pressure outflow boundary condition and an impermeable internal surface.

What the documentation says about the impermeable IS: “Two types of internal surfaces are available: Impermeable, which acts as a free-slip wall for both gas and solids phases…”

What the documentation says about the pressure outflow BC: “Outflow to a boundary at a specified constant pressure.”

How do these boundary conditions combine? To me it seems that if we would have constant pressure outflow boundary conditions, we wouldn’t need the internal surface. The two seem incompatible: one of them is an outflow boundary while the other one is a free-slip wall (open vs closed).

2

I suppose having the BC_3 region defined as a pressure outflow BC and an impermeable IS at the same time would give the same results as just defining it as a Free-slip wall BC alone, right? As documentation mentions:

“Two types of internal surfaces are available: Impermeable, which acts as a free-slip wall for both gas and solids phases, and Semi-Impermeable, which allows only the gas phase to pass through the internal surface.”

Unfortunately, it doesn’t give the same results. After some further testing it seems that having the BC_3 region with a pressure outflow BC behaves the same with or without additional impermeable IS. Could someone elaborate a bit on these boundary conditions, since I couldn’t find much clarification in the online documentation.