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-   -   Question about boundary conditions (http://www.cfd-online.com/Forums/fluent/43483-question-about-boundary-conditions.html)

Damiano January 26, 2007 05:28

Question about boundary conditions
 
Hi everyone,

I'm a fresh FLUENT user and I'd like to ask someone with more experience about a doubt that's roaming in my mind. I want to simulate the incompressible flow around a rotating inducer inside a cylindrical tube. The flow entering the tube is uniform with a known mass flow rate, the pressure is also known at the inlet. I want to calculate the mean pressure of the fluid at the exit, but I'm not sure on which boundary conditions to set for the inlet and outlet sections.

The best would be to set both velocity and pressure at the inlet but such a condition doesn't seem to exist in FLUENT. I tried to set a 'mass-flow-inlet' condition for the inlet and an 'outflow' condition for the outlet. Even if it seems to work from the standpoint of the pressure difference (the calculated value are rather close to the expected one), the absolute values of the pressure I get are rather strange (the inlet pressure is negative!).

The questions I'd like to ask are: 1) Are the boundary conditions I used consistent? 2) How can FLUENT calculate the pressure field without any boundary specification about it? 3) Are there different BCs that fit better the problem I'm trying to solve?

Any suggestion would be appreciate.

Thanks,

Damiano.

P.S. I'm using FLUENT 6.1.18


Seeker Phil January 29, 2007 14:56

Re: Question about boundary conditions
 
Hi, One thing you might want to check is your operating pressure condition and the location where you set it.

Also, check if you are reporting Gauge pressure, which might be possible for your problem

Phil

Damiano January 30, 2007 04:59

Re: Question about boundary conditions
 
Hi Phil and thanks for your reply,

in the Fluent's User Guide I read that operating pressure is not used in calculations with constant density fluids. However I tried to repeat the calculation setting several different values of operating pressure, but as expected I got every time the same solution.



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