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Boundary Conditions for a flow in pipes

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Old   March 22, 2014, 12:39
Default Boundary Conditions for a flow in pipes
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Masoud
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Hi dear friends

What is boundary conditions(inlet and outlet) that we should use in a compressible flow in a pipe and what is your reference? Kindly flow is subsonic.

regards

Last edited by masoudmohammadian; March 23, 2014 at 15:47. Reason: I made mistake because the flow is compressible not incompressible
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Old   March 23, 2014, 08:34
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sanam pudasaini
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Quote:
Originally Posted by masoudmohammadian View Post
Hi dear friends

What is boundary conditions(inlet and outlet) that we should use in a incompressible flow in a pipe and what is your reference? Kindly flow is subsonic.

regards
You can use either mass flow rate or velocity as inlet boundary condition and relative pressure as outlet boundary condition. But first set your reference pressure in order to use relative pressure.. Hope it helps
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Old   March 23, 2014, 08:57
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For inlet give velocity inlet condition and for outlet pressure outlet
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Old   March 23, 2014, 10:09
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Hi dear friend
Is it correct to use velocity when density is not constant? what is your reference of your answer?
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Old   March 23, 2014, 10:11
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thanks a lot my friend. I need the reference for your reply such as book, paper,...
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Old   March 23, 2014, 11:20
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Quote:
Originally Posted by masoudmohammadian View Post
Hi dear friend
Is it correct to use velocity when density is not constant? what is your reference of your answer?
You are asking for incompressible right? if it so then density is constant in that case.
For reference you can read boundary condition section in Fluent manual.
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Old   March 23, 2014, 15:46
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Thanks my dear friend
Sorry I made mistake because my flow is compressible

regards
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Old   March 24, 2014, 01:12
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Just a correction, for incompressible flow, density is not constant (atleast, not always).
Our (mu/rho) should be constant.
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Old   March 24, 2014, 02:15
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hi dear friend
I didn't get your answer

regards
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Old   March 24, 2014, 02:27
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What I meant to say is:
When you model incompressible flow, density is not always a constant variable. Its the kinematic viscosity(=dynamic viscosity/density) which is constant. For Low Reynolds number, flow most cases, the density'll not vary much.However, when you go to the moderate Re regime, most likely your density'll vary (depending of the fluid property though). So, as long as you keep kinematic viscosity constant, you are in "safe zone". Whereas keeping a constant density may lead to error.

that's all
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Old   March 24, 2014, 02:54
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but you know, my flow is compressible with Ma No. about 0.8
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Old   March 24, 2014, 03:12
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I was referring to the other post actually, where it was stated that incompressible flow=constant density..that's all(that's why i used "just a correction" phrase)
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Old   March 24, 2014, 03:17
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thanks a lot dude and sorry for any inconvenience
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Old   March 24, 2014, 13:54
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For Compressible flow through a pipe you can give Total Pressure at inlet and static pressure at outlet and from the stagnation static relations we can estimate the flow Mach number or the other way around if you want a particular Mach Number then calculate the relations accordingly with atmospheric pressure as reference.
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Old   March 24, 2014, 23:13
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Quote:
Originally Posted by som87 View Post
I was referring to the other post actually, where it was stated that incompressible flow=constant density..that's all(that's why i used "just a correction" phrase)
But numerically when we solve incompressible flow equation density term will be treated as constant and removed from momentum equations so i told him in terms of CFD(since he is working in it), but when comes to flow physics what u told is correct.
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Old   March 24, 2014, 23:18
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To add to my point:-
http://www.sciencedirect.com/science...21999108006748
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Old   March 25, 2014, 02:45
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OK, i don't think all solvers use this.
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