Hello,

I am modeling air flow in an apartment. flow comes in through an inlet section and goes out through an outlet section. I am prescribing fixed velocity at the inlet. What shoud be the boundary condition at the outlet - PRESSURE or MASS FLOW BOUNDARY ?

Jay

Hi Jay,

Probably a pressure boundary. The documentation discusses basic boundary condition selection, have a read.

Glenn Horrocks

Thanks Glenn,

I have read the section.

Actually, I have doing simulations for the past 2-3 months and I used Mass flow boundary instead of Pressure boundary. Are my simulations wrong then ?

Jay

Hi Jay,

The two out boundary conditions are different in most circumstances. I think a pressure outlet can be more parctical for your room simulation. If you are sure for the actual condition of the inlet and you can make a confessed decision. If you donot have many ideas, you can compare the two results of them for a more real-condition-like choose. Or maybe the two boundary conditions induce similar result for your interesting variables.

I got that PRESSURE B.C. condition are more suitable for outlet. But I have used mass flow b.c in all my simulations . Do I need to re-run all my simulations or should I continue to run my simulations using mass flow boundary ?

Jay

Since the pressure outlet may be more close to the real problem, may be you should change your boundary conditions. But I am not sure whether the change will improve the results or not.

Hi Jay,

What you have run should be OK. If your fluid was incompressible, your inflow velocity must result in the same mass flow rate as at the outlet, since the code has no ability to match the two. The main problem with what you have set up is related to convergence, since by fixing the flow rates you are severely restricting the mass equation.

Which boundary condition to use depends on what you are modelling. For the best performance, you should have at least one pressure bc, either total pressure at an inlet, or static pressure at an outlet or opening. Which to use depends on what assumptions you can make about the particular boundary conditions.

Regards, Robin

What will be the suitable condition for a mass flow boundary condition?