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CFX Begineer October 27, 2002 11:57

Inlet boundary condition
 
Hi everyone,

I hope to set two inlet boundary ocnditions for my model (static pressure & velocity profile). Is that possible? If so, how to do that? Thanks for your time and comments in this discussion.

lego October 27, 2002 14:52

Re: Inlet boundary condition
 
Would you explain more detailed?

My instinct say me, it is dangerous. What is (are) the outlet condition(s)?

LEGO

CFX Begineer October 28, 2002 00:13

Re: Inlet boundary condition
 
Hi,

Thanks for your interest. I'm modelling the flow behavior in the Francis turbine drafttube. I use the fully developed turbulent flow (Re=5x10^7) as a start and then apply the inlet velocity profile as the inlet BC and zero average relative static pressure as the outlet BC. However, I'm thinking if I could use put one more BC for the inlet (i.e. pressure).

Lego: Can you please tell me why this is dangerous?

lego October 28, 2002 03:35

Re: Inlet boundary condition
 
Huy!

Ok, sorry, I misunderstood you. I thoght that you wanted to define two inlets, one of them with pressure, one of them with inlet velocity. It would be dangerous, becaus in this case the resolution would very sensitively depend on the outlet boundary condition.

In your case, if you put both inlet velocity and pressure, the problem would be over definited.

The mass flow is a function of the pressure drop. You can not set the pressure drop, and the mass flow (i. e. velocity) in the same time.

What you can do, that first set the inlet boundary condition as inlet velocity, check the pressure drop between inlet and otlet, and then perform a new calculation with pressure inlet boundary condition with the proper pressure drop. Of corse, the mass flow won't be exactly the same, but you can make better the pressure drob iteratively.

LEGO

Neale October 28, 2002 17:25

Re: Inlet boundary condition
 
A static pressure inlet is dangerous because you are not saying anything at the inlet about the momentum flow. Hence, any momentum flow will do, and hence velocities can go all over the place and your solution may blow up/diverger/make no sense. You are much better off to set the total pressure which at least sets an upper bound on the inlet velocity.

You should only use a static pressure inlet when you know that the flow rate through the inlet boundary will be quite small relative to the mean flow. eg: say where the flow is highly tangential to the boundary condition.

Neale.

CFX Begineer October 29, 2002 00:01

Re: Inlet boundary condition
 
Hi Lego & Neale,

Thanks for your comments. Very useful to me. I will try that now and see what will happen then.

lego October 29, 2002 08:05

Re: Inlet boundary condition
 
Excuse me because of my english... Pleasa replace resolution by results...

So: I thoght that you wanted to define two inlets, one of them with pressure, one of them with inlet velocity. It would be dangerous, becaus in this case the RESULT would very sensitively depend on the outlet boundary condition.


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