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-   -   How to realize fixed pressure in my codes? (https://www.cfd-online.com/Forums/phoenics/51573-how-realize-fixed-pressure-my-codes.html)

 Leon March 13, 2003 17:13

How to realize fixed pressure in my codes?

Hi,all,

I am coding myself and using the method of Phoenics to deal with the bcs: source S=[cphi+cm(vm-p)][vphi-phi]

to fix outlet pressure:in phoenics it is suggested to make cm=1000*expected flow rate/T, vm=0.0 then the incell pressure can be set to fix pressure of 1e-3 order of the vm(actually zero here).

Should I put this mass source into P' equation then I will get incell pressure fixed?

Another question is: when outflow rate is known for example 1m3/s how can I introduce this "source" in the momentum equation by using S=[cphi+cm(vm-p)][vphi-phi]?

When outflow occurs the source S changed into S=cphi(vphi-phi) only and usally the cphi is zero so does that mean no source will be needed to add into the momentum equation for outlet cells?

Thanks a lot for your help and patience.

 cham SUPPORT March 14, 2003 06:26

Re: How to realize fixed pressure in my codes?

The expression dSm/dP should go into the ap (central coefficient) of the pressure-correction (p') equation, where Sm=cm*(vm-Pp)*T and T is is the source type, i.e. area, volume or cell(unity). Then a large value of Cm will act to suppress pressure corrections and so fix the pressure in the cell.

Yes, if there is outflow from the cell for the variable phi then nothing need be added to the phi equation. This follows from use of inflows=outflows (continuity) when deriving the finite-volume form of the equation for phi. This form ensures that -m*phi is taken from the cell when there is outflow.

 Leon March 16, 2003 01:23

Re: How to realize fixed pressure in my codes?

Hi,cham SUPPORT:

Thanks so much!I have tried this for duct flow and it works pretty well.

But: When the pressure is "fixed", small changes resulting from dynamic- head variations can change inflow to outflow and vice versa, with consequent convergence difficulties.

How phoenics deal with this problem?Any magical tricks?(this problem is significant when fixed pressure is applied to large openings where inflow and outflow both exist.) During every iteration when incell pressure is smaller than the outside pressure then this cell should be taken as an inflow cell BC for this time iteration,right? To make the question clearer: For fixed pressure case how to deal with the situation where both inflow and outflow occurs in single opening in Phoenics?