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-   -   Inlet and outlet boudary without wall between (http://www.cfd-online.com/Forums/cfx/96815-inlet-outlet-boudary-without-wall-between.html)

Janshi February 1, 2012 17:19

Inlet and outlet boudary without wall between
 
1 Attachment(s)
Hello,

I have following model:

A rectangular box with an Inlet and outlet. The sides as well as the top and the bottom are defined as wall.
The inlet as well as the outlet boundary conditions are defined via a velocity profile. The velocity profile is defined normal to the boundary.

Following changes are to be done:

The velocity profile definition is to be changed from a normal to the boundary definition to a definition via cartesian componentns. The velocity profile should be defined angular to the boundary.

Ive consider following changes in the setup:
Can I change the definition for the sides into inlet and outlet boundaries?
In the attachment Ive made a primitive sketch of my idea. Im quite sure that there wont be any recirculation flow, so could such a case calculated in cfx?
Actually Im not sure, because Ive never tried a calculation where no wall exists between the inlet and the outlet of the flow domain.

Thank you

ghorrocks February 1, 2012 18:30

It is not a good idea to have an inlet and outlet so close. This looks numerically unstable to me. I would move your boundaries further back to where they are separated and simpler.

Janshi February 1, 2012 19:33

Hello glenn,

First, thank you for your fast answer.
I already assumed that there would be problems. Ive already given it some tries but every calculation blow away after some iterations depending on the timestep Ive chosen. Usually my Mach number grows ridiculous high (beyond 10) and the the calculation blow away with a fatal error that the linear solver diverged.

I assume that if I define the sides as openings, nothing will change at the problem with numerical stability?


Best regards
Shichun

ghorrocks February 1, 2012 19:51

No, that will not improve numerically instability.

You might be able to get it to work, first try a smaller time step.

Why are you doing it like this? What are you trying to do? Are you trying to put the central object under different angles of attack without having to remesh?

Janshi February 2, 2012 04:08

Quote:

Originally Posted by ghorrocks (Post 342362)

Why are you doing it like this? What are you trying to do? Are you trying to put the central object under different angles of attack without having to remesh?

Yes thats the idea. For special reasons Ive not the possibilities to remesh and I have to leave the mesh untouched :|

ghorrocks February 2, 2012 05:51

Have you considered a C grid like this http://en.wikibooks.org/wiki/ICEM_CFD ?

They are frequently used in AOA sweeps. You make the circular, top and bottom surfaces inlets and the back face an outlet.


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