Boundary Conditions for a flow in pipes
 

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

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 :)

For inlet give velocity inlet condition and for outlet pressure outlet

Hi dear friend
Is it correct to use velocity when density is not constant? what is your reference of your answer?

thanks a lot my friend. I need the reference for your reply such as book, paper,...

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.

Thanks my dear friend
Sorry I made mistake because my flow is compressible

regards

Just a correction, for incompressible flow, density is not constant (atleast, not always).
Our (mu/rho) should be constant.

hi dear friend
I didn't get your answer

regards

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

but you know, my flow is compressible with Ma No. about 0.8

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:o)

thanks a lot dude and sorry for any inconvenience

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.

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.

To add to my point:-
http://www.sciencedirect.com/science...21999108006748

OK, i don't think all solvers use this.