[Jack Hayton]

I am simulating flow around the fuselage of a RC drone. The drone uses a hydrogen fuel cell that requires active cooling. There are some openings in the side/bottom of the fuselage where the air enters and is routed to the fuel cell that sits in the rear of the fuselage. The back of the fuselage is open and is where the cooling fans of the fuel cell push the air out. This area is highlighted purple in the geometry image I have attached.

So far I have successfully simulated the flow with no openings in the fuselage (everything is a wall). I have attached an image of the velocity profile around the fuselage (Side view) of said simulation. In this image it is clear that there is flow separation/recirculation on the flat rear face. Intuitively I would think that this area of recirculation would be reduced/produce less drag when taking into account that air is being blown out of the rear surface, this is what I want to test.

My question: What boundary condition should I set this back surface too? Velocity inlet or mass flow inlet? I believe it is an inlet as flow is entering my flow domain from this surface (I know its confusing as its the "outlet" on the fuselage).

P.S. For the time being I am not going to include the "cooling inlet" holes on the side/bottom of the fuselage, but I do plan on adding these later. Due to the complex interior shape of the ducts/fuel cell I will not model them using volume mesh but simple add outlet boundaries onto the outer fuselage surface. What boundary condition should I use for these outlets? I am using STAR CCM+.

Thanks in advance,
Jack