[hamed.majeed]

Hi

I want to fix my static pressure at the inlet of the domain. There is no inlet boundary condition (as fluent names it) that does that.

Can I use a pressure outlet boundary condition at the inlet instead?

What would be the implications of doing so...other than to be careful with the direction to avoid Fluent flow reversal errors!

[LuckyTran]

It's highly non-physical. The only case where it makes sense to fix the static pressure at the inlet is if the flow is super sonic (and the pressure inlet BC does do this), which is also the implication if you do choose to fix the static pressure. Eventually you will run into another inconsistency. Fluent intentionally doesn't support this boundary condition because it doesn't make sense to do something that doesn't make sense.

[hamed.majeed]

Originally Posted by LuckyTran

It's highly non-physical. The only case where it makes sense to fix the static pressure at the inlet is if the flow is super sonic (and the pressure inlet BC does do this), which is also the implication if you do choose to fix the static pressure. Eventually you will run into another inconsistency.

Here is a what-if scenario!

I have an incompressible flow through a pipe.
At inlet I fix the static pressure, hence, a "pressure-outlet" boundary condition.
At outlet I fix the velocity, hence, a "velocity-inlet" boundary condition.

Is this still a non-physical problem? I assume this is a well-posed situation!

[LuckyTran]

Originally Posted by hamed.majeed

Here is a what-if scenario!

I have an incompressible flow through a pipe.
At inlet I fix the static pressure, hence, a "pressure-outlet" boundary condition.
At outlet I fix the velocity, hence, a "velocity-inlet" boundary condition.

Is this still a non-physical problem? I assume this is a well-posed situation!

The velocity outlet BC is still non-physical because it is a uniform velocity constraint. The hand of God reaches into your simulation and imposes a uniform velocity field at the exit, which doesn't happen in real life (the velocity is whatever it wants to be). This miracle happens while you are solving a boundary value problem pde. Is it mathematically ill-posed? No. But it still requires some divine intervention for any real flow to behave that way. If you choose to use that set of BC's, then that is the implication carried with your result.

Why not take it a step further and impose a static pressure and velocity at the inlet and have no boundary condition at the outlet? Think about it...

I get that sometimes you know all the thermodynamics entering a simulation and want to impose it. But thermodynamics is ignorant of the forces that causes a system to be at or arrive at a state. If you run into a situation where you need to force a static pressure at the inlet, then you are thinking in the thermodynamic mindset and ignoring physical context. You're using the wrong tools for your problem.


Can you do it? Yes. But I'm strongly recommending you to figure out how else (i.e. the proper way) to approach the problem.

[hamed.majeed]

Thank you...appreciate your response