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Outlet BC for subsonic flow in pipe

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Old   May 9, 2003, 10:07
Default Outlet BC for subsonic flow in pipe
  #1
andrea panizza
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Hello, I would like to ask your help on the following case. I have a pipe with an internal obstacle placed on the symmetry axis, a small disc. The regime is subsonic compressible laminar flow and the material is air. I know pressure, temperature and mass flow rate at the inlet, and I need to calculate the pressure drop through the system. This poses a problem for boundary conditions. At the inlet I use "mass flow inlet", and that's fine. At outlet I would like to use a "fully-developed flow" boundary condition, e.g. "outflow" boundary condition, since I cannot prescribe the outlet pressure, which is what I need Fluent to compute. Unfortunately, Fluent manual says I cannot use "outflow" boundary conditions for compressible flow, and suggests "pressure outlet" instead. Have you got any suggestions? Thanks, Andrea
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Old   May 9, 2003, 10:54
Default Re: Outlet BC for subsonic flow in pipe
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Laika
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If you have a mass inlet and a compressible gas, you need some pressure information to have a well-defined problem. How else can Fluent find the density and the velocities?

Why can't you say anything about the outlet pressure?

Laika, stil orbiting
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Old   May 9, 2003, 11:59
Default Re: Outlet BC for subsonic flow in pipe
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Adarsh
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if you know the mass flow rate and pressure at inlet then u can use "pressure inlet BC" with "pressure outlet BC PLUS Target mass flow rate option" .

This way the pressure will fluctuate at the outlet till the desired mass flow rate is obtained.

Read Fluent manual for TMR option . this is given through Text user interface. hope this helps

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Old   May 9, 2003, 12:12
Default Re: Outlet BC for subsonic flow in pipe
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andrea panizza
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I thank you very much for your interest in my problem and your help.

As I told you in my preceding post, the objective of my computation is the pressure drop through the system corresponding to a given mass flow rate. Since I know the inlet pressure, I cannot know the outlet pressure, otherwhise I would not need to do any computation.

I do not agree with you that problem is underconstrained, though: not to set a value of outlet pressure does not mean that I give no BC on the pressure. It means that I do not give a Dirichlet BC on the pressure. But I could give a Neumann BC: physically, this amounts to saying that pressure, velocity, etc. at the outlet cross section all have the same value thay had at the cross section which immediately precedes outlet. This would be the "outflow" BC in Fluent, but it cannot be used in compressible flow computation.

I do think that pressure drops through a system in compressible flow must be computable with Fluent 6.0.20 , but how?

Cheers,

Andrea
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Old   May 9, 2003, 12:22
Default Re: Outlet BC for subsonic flow in pipe
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andrea panizza
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Thank you very much, Adarsh, this is exaclty what I needed. If the option is usable for compressible flow, then I am done. I immediately go and look in the manual this TMR option.

Thank you again,

Cheers,

Andrea
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Old   May 10, 2003, 06:54
Default Re: Outlet BC for subsonic flow in pipe
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Andrea Panizza
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Dear Adarsh,

I searched through the pdf manual and I also looked in the "pressure outlet" panel of Fluent, but I didn't find any reference to Target Mass flow Rate. Fluent version in use at my university is 6.0.20. Are you sure the TMR option is available in this version, or pheraps is it a feature of last version (6.1) only?

Thank you again,

Andrea

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Old   May 10, 2003, 07:44
Default Re: Outlet BC for subsonic flow in pipe
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Jonas Larsson
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Yes, it might only be available in 6.1, and even in 6.1 it is only available through the text interface.

However, there are scheme files for both Fluent 5 and 6.0 available to do this (Search the support database at http://www.fluentusers.com , or grab a version from ftp://ftp.cfd-online.com/fluent/ )

However, using a mass-flow inlte and a static pressure outlet with an estimated outlet pressure (based on an estimated loss and your known inlet pressure), should give you a very good figure for the loss. You'll not get exactly the right inlet pressure, but a small difference in pressure level in your simlation compared to reality doesn't affect the losses much as long as you have the correct mass-flow.
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