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-   -   pressure outlet BC for incompressible flow (http://www.cfd-online.com/Forums/main/10055-pressure-outlet-bc-incompressible-flow.html)

 khaiching October 10, 2005 05:23

pressure outlet BC for incompressible flow

Hi: I have implemented the SIMPLE method by Patankar (1980) to compute incompressible flow. To correct the pressure field, the so-called Pressure-Correction Equation has been employed.

I am in the midst of searching the correct implementation of BC at the boundaries (inlet, outlet, wall) for pressure correction terms (p'). At outlet faces, Constant pressure (p) has been used and thus the p' is set to 0 while extrapolating the 3 velocity components; for other boundaries, p and p' are extrapolated.. Unfortunately, the solution diverges !!

Is my treatment correctly implemented? Could anyone help to advise on the correct implementation of pressure outlet BCs (fixing the P) for SIMPLE algorithm??

I have tried to scale the mass flow rate at outlet boundary. It works; however, this method needs prior knowledge of the outlet mass flow rate; which I think it is impractical for unsteady flow..What you guys think??

-khai ching-

 Quarkz October 10, 2005 05:41

Re: pressure outlet BC for incompressible flow

hi,

maybe u can try using under-relaxation mtds for your pressure & maybe velocities as well. this should prevent divergence. recommended values from fluent are 0.3 & 0.7 respectively. hope it helps

 khaiching October 10, 2005 05:49

Re: pressure outlet BC for incompressible flow

I have implemented 2 methods to treat the outlet BCs

(1) Pressure outlet: fixing the pressure (p' = 0) and extrapolate the velocities.. I have tried to reduce the u/relaxation factor to about 0.05 for pressure and 0.10 for velocities; However, it fails...:(

(2) mass scaling at outlet: scale the velocity vectors at outlet as accordance to the expected mass flow rate.. This works..I have extrapolated p' and P at all boundaries..

I am wondering whether the Pressure Correction Equation can be implemented together with the pressure outlet BC.. From Davidson (1996), he has extrapolated p' and P at ALL boundaries..

-khai ching-

 Adrin Gharakhani October 10, 2005 15:00

Re: pressure outlet BC for incompressible flow

>> I have tried to scale the mass flow rate at outlet boundary. It works; however, this method needs prior knowledge of the outlet mass flow rate; which I think it is impractical for unsteady flow

If I understand you correctly, you extrapolate your velocities to the boundaries and then scale them using conservation of mass. The extrapolation is probably ok (there really is no good exit boundary condition, without having to make some sort of simplification/assumption). My response is particularly related to the scaling issue. There is nothing wrong with scaling, and indeed you may have to do it anyway to make sure you conserve mass. Your concern about the need to know the mass flow rate in advance seems unjustified since you're dealing with incompressible flow. Therefore, unsteady or not, you already know the exist mass flow rate based on your inlet (and wall) boundary conditions.

 khaiching October 13, 2005 22:48

Re: pressure outlet BC for incompressible flow

Dear Adrin: Does that mean I must scale the mass flow rate at outlet even I impose the constant Pressure BC at outlet?

I am using the Pressure Equation right now instead of the pressure correction equations. I am still in the midst of verifying my implementation..

-khai ching-

 Adrin Gharakhani October 14, 2005 14:42

Re: pressure outlet BC for incompressible flow

Just want to make myself clear what I mean by scaling: you have an input mass flow rate m_i, and you get a solution that has an exit velocity distribution u_e. Now if you multiply u_e by the local density and cross-sectional area, chances are you'll get an exit mass flow rate m_e, which is not exactly equal to m_i. (hopefully the two are not too far off; if so, then you have an error some place). Then you want to scale your u_e such that m_e=m_i. This "scaling" has nothing to do with your BC type; it is there to make sure that you don't have source accumulation/depletion (by violating continuity)

I don't know the details of your approach, but make sure you don't over/under-specify your boundary conditions (i.e., have more BCs imposed than is necessary as dictated by the pde)

 khaiching October 15, 2005 02:58

Re: pressure outlet BC for incompressible flow

thanks..

My code works right now with the pressure outlet BC implemented and continuity is satisfied using the Pressure Equation..(Thanks to Dr Hrvoje Jasak)

I still have some doubts about the implementation of Pressure Correction Equations(PCE). Anyone has used the PCE in concert with the constant pressure outlet BC? What will be the boundary conditions (p') at the boundary faces? So far, I have extrapolated p' at all boundaries while fixing p'=0 at outlet(since I fix P_out). This method dint work unless I extrapolated p' and P at outlet as well. By doing that, I have to scale the mass flow rate at outlet to ensure global mass conservation..

-khai ching-

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