Buoyancy Analysis: Fluid Surface Boundary Condition
 

Hello,

I hope I'm posting this in the right place. I am trying to model a tank full of fluid, I am starting with water since its properties are readily available. The walls of this tank are fixed and the top surface is exposed to air, which I will give a convection coefficient and an air temperature. The bottom surface is also given a "convection coefficient", a U-Value, along with a ground temperature.

What I am interest in seeing is how the heated walls create a buoyancy effect and the convection of the fluid you would get inside the tank as well as the temperature distribution throughout the fluid. All this to ask, in what way can i define the top surface? It is kind of like an opening but then I cannot specify a convection coefficient plus the fluid will just rise up and out of my tank and if i put a wall there I get an overflow error which i assume is because the water expands due to thermal expansion but has no where to go since i've mad every boundary condition a closed wall.

Could any advise on how to handle this? I know i could just model the air but that just seems like extra computation for something i'm not all that concerned about and figure there has to be a way around it.

Thanks in advanced for any help anyone can give!

Cheers,
Giovanni

In other words, you don't know the heat flux to set as Neumann condition at the surface?