This may be a trivial problem, but I'm having a tough time with conceptualization.
I'm using a code for modeling flow out of a tank. I'm not experienced in fluid dynamics; I'm more experienced with chemical aspects of the problem. I want to use pressure boundary conditions (a constant pressure at the bottom of the tank and atmospheric pressure at the tank outlet). The code is for incompressible flow and I must input the pressure values in terms of pressure/density (units of m2 s-2).
The problem is I don't understand which densities to use for each boundary condition. At the bottom of the tank, I'm guessing that I should use the tank fluid density (or some weighted average if there is more than one kind of fluid layer in the tank). But what should I use at the top (where the boundary condition is the pressure of the atmosphere)? When I divide atmospheric pressure by the density of air, I get a value that is higher than the bottom boundary of the tank and flow goes the wrong way (down, not up to the outlet). When I divide atmospheric pressure by the same density that I use for the bottom boundary (the density of the fluid in the tank), my output shows pressure values that are lower than atmospheric. It seems like I should get this, but I don't. Any help would be appreciated, but unfortunately, I'm not allowed to post the code.