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-   -   B.C.S on outflow outlet and pressure outlet (https://www.cfd-online.com/Forums/main/15238-b-c-s-outflow-outlet-pressure-outlet.html)

 kenneth May 28, 2008 22:43

B.C.S on outflow outlet and pressure outlet

Hi all,

In my code I always apply the outflow boundary condition (zero normal gradients for all variables) to the outlet subject to fully-developed flow condition. Another boundary flow condition for the outlet is pressure outlet. However, I have no idea about the second b.c. in theory and implementation in a CFD code. But my recent work need the use of the second one in my code. I would appreciate your hints.

Cheers,

Kenneth

 Harish May 29, 2008 06:55

Re: B.C.S on outflow outlet and pressure outlet

Look for the paper of Poinsot and Lele (1992 or 1991), Journal of Computational Physics for a detailed description.

 Vinayender May 29, 2008 07:49

Re: B.C.S on outflow outlet and pressure outlet

Hi,

Certainly even I suggest you to read the paper what Harish has suggested first,

But I have some thing to share with you in this regard. This pressure oultet BC (which is valid only for subsonic flows) can be implimented as follows.... Just use the pressure value on the outflow boundary value and other variables can be extrapolated form inside. This is what i have done as one of the method calculate fluxes on subsonic outflow boundary for my Finite volume compressible flow solver.

This is because for subsonic flow two eigen values (u, u+a) will be positive and one (u-a) will be negetive, means two characteristics waves from the boundary will be moving inside the boundary and one characteristic wave will be moving out from the boundary face, to say this roughly one variable value should be calculated from outside boundary (say pressure)and other variables should be calculated form interior zone.

Thanks, Vinayender.

 kenneth May 29, 2008 09:54

Re: B.C.S on outflow outlet and pressure outlet

Thanks. I currently can not access the paper, but can study the recommended paper tomorrow. It sounds that pressure outlet b.c. is more related to compressible flow. What I have involved in the past and present are all incompressible flows. I notice that Fluent can specify pressure-outlet or outflow-outlet b.c. for most of applications, e.g. pipe flow at a very low velocity, and the pressure-velocity coupling can be based on SIMPLE. So how this has been implemented (e.g. for a SIMPLE based solver applicable to incompressible flows)?

 kenneth May 29, 2008 20:57

Re: B.C.S on outflow outlet and pressure outlet

The paper mentioned above is dealing compressible flows. I need to know how to implement pressure outlet for incompressible flows in a code based on pressure correction equations(SIMPLE by Patankar). This will be applied to the cases where I do not know flowrates at the outlets.

Regard,

Kenneth

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