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-   -   Which type of boundary conditions ? (http://www.cfd-online.com/Forums/fluent/92713-type-boundary-conditions.html)

 cpinz September 22, 2011 16:11

Which type of boundary conditions ?

Hi all,

I'm analyzing a wind tunnel like duct; air is moved by a fan placed a the exit with a given mass flow rate. At inlet I have environment pressure and zero velocity.

I wonder which are the correct boundary conditions for such case.
Help is appreciated.

Thank you

 m2montazari September 23, 2011 03:11

hi,
if you have a duct, you cant have zero velocity at the inlet. zero velocity is for infinitely upstream condition. I dont know what is you problem and domain but if you know the pressure drop between inlet and outlet of duct(just before the fan) you can use pressure inlet and outlet. but if not, if you know mass flow rate, use mass flow inlet and pressure outlet. also if the problem is incompressible and the inlet is far enough from the body in the tunnel, you can use velocity inlet at inlet with V=Q/A and a boundary layer would appear and grows at the beginning of the duct.
yours,

 cpinz September 23, 2011 05:04

The domain is very simple: a tube and ,as I said, you can think about it like a windtunnel.
Since the flow is driven by a fan placed at the end (outlet) of the tube, I suppose that at the inlet I have the following condition:
-flow is at rest at inlet
- pressure at inlet equals pressure of environement.

I know the mass flow rate at the fan and on the basis of conditions above I need to evaluate the flow speed at some section along the tube.
But I'm a little confused about which boundary condition , available in Fluent, I can use.

Mass flow rate is available at inlet and not at outlet as I would like..:rolleyes:

 m2montazari September 23, 2011 06:37

hi,
by continuity it is obvious that mass flow rate at outlet is exactly equal to mass flow rate at inlet. so at inlet there is no rested air. as I previously said rest is for infinitly upstream, not for inlet.
simply use "mass flow inlet" for inlet and "pressure outlet" for outlet
yours,