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-   -   No sharp interface - Discharge in a pipe (http://www.cfd-online.com/Forums/flow-3d/27256-no-sharp-interface-discharge-pipe.html)

 Matthieu Breysse October 21, 2008 08:35

No sharp interface - Discharge in a pipe

Hello dear users,

I am trying to calculate a discharge in a pipe with a fixed pressure and flow. What are the best boundary conditions at the entrance section and the exit section (that is to say at the begining and at the end of the mesh)? When I impose the velocity at the begining and at the end the pressure field don't stabilize and I also get negative pressure. What is the best limit condition to impose on this problem ?

Thanks for helping me.

Matthieu

 HC October 21, 2008 11:07

Re: No sharp interface - Discharge in a pipe

I assume you meant that you got pressure iteration failures by saying the pressure filed don't stabilize. For that problem, you may need to check the mesh, or increase the maximum number of iterations, or use a bigger GMRES subspace size, e.g. 15, if GMRES pressure solver is applied. You may also need to use a small initial time step.

As you used velocity boundary conditions at both inlet and outlet, there was no reference pressure given in the domain and the pressure result you got would just be gauge pressure. So a negative pressure in that case would be ok.

A typical boundary set for pipe flow would be velocity/volume flow rate inlet and fixed pressure outlet (as you mentioned).

 Michael Barkhudarov October 21, 2008 23:46

Re: No sharp interface - Discharge in a pipe

Matthieu,

Having velocity at both ends of the pipe is overspecifying the problem. This causes pressure convergence problems and unphysical pressure values. Do you get a message from the mentor about a net in/out flow in the domain?

Having velocity at one end, e.g., inlet, and pressure at the other should do the trick.

Michael

 Matthieu Breysse October 22, 2008 12:09

Re: No sharp interface - Discharge in a pipe

Thanks for your advice, I put the same simulation with the velocity at the entrance section and the continuative condition at the exit section and it is working.

Matthieu

 Michael Barkhudarov October 23, 2008 09:26

Re: No sharp interface - Discharge in a pipe