[Christ]

That can only happen if the flow had greater pressure than the ambient on the opposite side of the membrane. If the pressure of the flow is equal, turbulence should create a boundary layer. If the difference is negative (highest purdue on the non-flow side), there should be an extraction of purdue as the flow passes each hole, which, by itself, will keep the flow from actually making contact or being able to permeate the surface.
Perpendicular, obviously the flow will have more force against the membrane than if it were parallel, so a higher ambient pressure behind the membrane would be necessary to prevent flow through.