# Boundary Conditions for a flow in pipes

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 March 22, 2014, 12:39 Boundary Conditions for a flow in pipes #1 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 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

March 23, 2014, 08:34
#2
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sanam pudasaini
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Quote:
 Originally Posted by masoudmohammadian 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

 March 23, 2014, 08:57 #3 New Member   Vishnu C Join Date: Oct 2013 Posts: 22 Rep Power: 5 For inlet give velocity inlet condition and for outlet pressure outlet masoudmohammadian likes this.

 March 23, 2014, 10:09 #4 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 Hi dear friend Is it correct to use velocity when density is not constant? what is your reference of your answer?

 March 23, 2014, 10:11 #5 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 thanks a lot my friend. I need the reference for your reply such as book, paper,...

March 23, 2014, 11:20
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Vishnu C
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Quote:
 Originally Posted by masoudmohammadian 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.

 March 23, 2014, 15:46 #7 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 Thanks my dear friend Sorry I made mistake because my flow is compressible regards

 March 24, 2014, 01:12 #8 Member   Join Date: May 2012 Posts: 68 Rep Power: 6 Just a correction, for incompressible flow, density is not constant (atleast, not always). Our (mu/rho) should be constant. __________________ Best wishes, Somdeb Bandopadhyay

 March 24, 2014, 02:15 #9 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 hi dear friend I didn't get your answer regards

 March 24, 2014, 02:27 #10 Member   Join Date: May 2012 Posts: 68 Rep Power: 6 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 __________________ Best wishes, Somdeb Bandopadhyay

 March 24, 2014, 02:54 #11 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 but you know, my flow is compressible with Ma No. about 0.8

 March 24, 2014, 03:12 #12 Member   Join Date: May 2012 Posts: 68 Rep Power: 6 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) __________________ Best wishes, Somdeb Bandopadhyay

 March 24, 2014, 03:17 #13 New Member   Masoud Join Date: Feb 2014 Posts: 21 Rep Power: 5 thanks a lot dude and sorry for any inconvenience

 March 24, 2014, 13:54 #14 Member   Ramesh K Join Date: Dec 2009 Location: Bangalore Posts: 65 Rep Power: 9 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.

March 24, 2014, 23:13
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Vishnu C
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Quote:
 Originally Posted by som87 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.

 March 24, 2014, 23:18 #16 Member   Join Date: May 2012 Posts: 68 Rep Power: 6 To add to my point:- http://www.sciencedirect.com/science...21999108006748 __________________ Best wishes, Somdeb Bandopadhyay

 March 25, 2014, 02:45 #17 New Member   Vishnu C Join Date: Oct 2013 Posts: 22 Rep Power: 5 OK, i don't think all solvers use this.

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