[AndrewP]

Hi Everyone,

I'm having an internal debate about the best boundary conditions for a buoyancy-driven flow problem I'm working on. This seems like it should be a standard/textbook problem with a well-accepted answer. Sorry fi this is obvious, my background is finite-element of solids not fluids, so I have limited experience with CFD.

I have a room with a series of open windows and a chimney. I'm trying to calculate the flow field in the room to estimate the transport of a minor gas species. The transport part of my model works. My first iteration of the model set the pressure on all the openings at the hydrostatic gas pressure (P=1 atm-density*gravity*vertical_coordinate). A corresponding volume force was applied based on the gas density. I found that this version of the model runs well for a while but seems to get some positive feedback on the velocity, and I'm not sure that is physically real (it is still a matter of debate).

I thought maybe the pressure at the inlets should be reduced according to the flow velocity. The logic here is that if the air is moving it must have already been through a pressure gradient. So I tried changing the pressure boundary condition to Bernoullii's Principle. So it looks like P=1 atm-density*gravity*z-1/2*density*magU^2.

But thinking about it more I'm still not very happy with this when it comes to outflow. If the gas is outflowing the window should see a higher pressure to push other air out of the way. But the Bernoilli expression above would see a lower pressure for the outflow. That makes me think I should have a flow resistance term instead (or in addition to). But that is getting pretty messy and I'm less certain about the theoretical justification.

One way to approach this would be to CFD a volume outside the room, but I'm assuming there is a more elegant way. What BCs would you recommend? Or should I just model the outside?