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 Rbm September 1, 2000 07:47

Pressure Boundary Condition

I am a new comer. Can any one tell me about what should be the pressure & velocity boundary conditions in the exit & entry of a control volume, while dealing with Patankars SIMPLE method.

 Sebastien Perron September 1, 2000 08:27

Re: Pressure Boundary Condition

1) Entry of a duct: imposed velocity, pressure unknown neumann boundary condition. 2) Exit of a duct: velocity unknown (0 neumann boundary condition ), pressure imposed and equal to 0.

 eddy September 1, 2000 20:56

Re: Pressure Boundary Condition

If there is backward inflow at outflow boundary, how to deal with that situation?Can we put Pressure to zero?

 Sebastien Perron September 2, 2000 08:39

Re: Pressure Boundary Condition

How is this inflow generated? Usually when you deal with forced flow (such as flow in a duct, fow over a backward facing step, flow past an object), your domain is large (long) enough that the outflow is not supposed to influence the flow within the domain. This is a free exit and this is why we put the pressure to zero (when dealing with incompressible flow).

If you send me a description of the problem (.ps .pdf), I may be able to give more help.

 D.M. Lipinski September 2, 2000 16:23

Re: Pressure Boundary Condition

One can have a local inflow through an outflow boundary when you place the outlow not far enough. For the backward facing step example, this would be to place the outflow boundary within the region of the recirculationg flow behind the step. You can still solve this problem provided that you satisfy the integrability condition of the pressure correction equation. This condition requires the surface integral of the normal pressure derivative be equal to the sum of mass flow rates (global mass error). To satisfy the above condition one must set the correct boundary conditions for both momentum and pressure equation and rigorously request the mass balance within each outer iteration of SIMPLE. For the inflow part on the outflow boundary, one should set the Dirichlet boundary conditions (for a single outer iteration). Inflow properties (density etc.) may be taken from the nearby element. The inflow/outflow parts of the assumed outflow boundary adjust themself within several outer iterations. The above works with both staggered and cell-centered finite volume schemes. For more description c.f. Blosh, Shyy, Smith Numer.Heat Transfer B, 24(1993)pp.415-429 (for the staggered grid scheme). The cell-centered scheme is more tricky, but it may be done as well.

regards

DML

 eddy September 2, 2000 16:45

Re: Pressure Boundary Condition

Actually, there are a lot of computational cases, in which there is backward inflow at outflow boundary. One of this is case is the unsteady simulation of wave prorogation. I have got one paper about 'On the open boundary condition for the simple algorithm using nonstaggered grids', Yih Nen Jeng and Yuan Chang Liou 1994 , Numerical Heat Transfer Part B. It may help.

 JaeWonLee September 14, 2000 09:41

Re: Pressure Boundary Condition

Can any one tell me, what the exact definition of pressure boundary condition? How can I put the pressure boundary condition with PISO algorithm?

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