Hello,

Can anyone tell me the difference between the two boundary conditions; the static pressure boundary condition and the Opening pressure boundary condition. I have tried to understand them from the explaination in CFX HELP but couldnt get the idea clear. Given examples for both cases will make it easier for me to understand it. Thanx alot for your help and time. Best Regards

I'm not sure if I fully understood your question. Do you mean by 'static pressure BC' a 'inlet BC'? Beside others, you can chose in CFX between inlet, outlet, opening, and wall BC. First two allow only in or exit flow, respectively. If you chose e.g. an outlet BC and the solver computes some recirculating flow, the code will place a wall (between 0 and 100%) at the exit to prevent the fluid from entering the domain again. The 'opening BC' on the other hand can be used if you don't know a priori the flow direction at the boundary. That includes for instance the case where you actually could have a flip of the flow direction during the solution process (e.g. during a transient run). As you have probably also noticed, you have plenty of possibilities at your hand to adapt the particular BC (e.g. opening BC) to your conditions (e.g. static pressure and direction, static pressure for entrainment, ...).

Hope this helps! R, Philipp

Thanx Mr. Beierer, Now things are clearer, but I should have made my question as the following. What is the difference between Opening Pressure and Static Pressure in the Opening boundary condition. Do they both allow in and out? The problem Im trying to solve is that I need to model a huge room, but in order to make the computation faster, I am modelling it smaller in hight direction, and instead of the cieling (Wall) I use an opening boundary condition. so is this boundary condition here the best choice? or do you think an outlet will do better? Thanx for your time. Best Regards Zurayk

The value you set for Opening Pressure will define the TOTAL pressure for flow entering the opening and STATIC pressure for flow exiting the opening.

The value you set for Static Pressure will define the STATIC pressure for both flow entering and exiting the opening boundary.

Opening pressure is more generally more robust. Both allow flow in and out of the domain.

While modelling the entire room is probably the best thing to do from the point of accuracy, using an Opening sounds to reduce the problem size sounds ok for your case. If all of the flow exits through the boundary then you could use an outlet instead.

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