# Vacuum condition

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 March 4, 1999, 08:10 Vacuum condition #1 Samson Yim Guest   Posts: n/a Hi, I am working on a problem of a fluid flowing into a vacuum chamber. Can anybody suggest me how to setup the vacuum (initial) condition in CFX? Thanks in advance!

 March 4, 1999, 12:16 Re: Vacuum condition #2 Alessandro Grossi Guest   Posts: n/a Hi Yim, I you may use this: >>MODEL DATA >>SET INITIAL GUESS >>SET CONSTANT GUESS PRESSURE 0.0000E+00 I hope this helps

 March 4, 1999, 13:13 Re: Vacuum condition #3 Samson Yim Guest   Posts: n/a Thanks for your suggestion. Furthering my question. Does the liquid-vacuum system considered as one-phase or two-phase system? If it is a two-phase system, what would you put in for the physical properties of vacuum condition (density and viscosity)?

 March 4, 1999, 18:51 Re: Vacuum condition #4 Glenn Horrocks Guest   Posts: n/a Hi, I think you're going down the wrong track by running the problem 2 phase. I also think that you will have a problem with setting the initial guess of pressure to be zero - I'm sure CFX won't like that, as PV=mRT no longer holds. What I would do is run with an initial guess of a very small pressure (say 1 Pa, 0.01 Pa, whatever) and run it as a normal compressible gas problem. Just watch out for the reference pressure under >>COMPRESSIBLE OPTIONS, REFERENCE PRESSURE; and make sure that the minimum allowable pressure and temperature is not getting in the way under MINIMUM REAL PRESSURE and MINIMUM TEMPERATURE. You may also find that the default values of viscosity, specific heat, equation of state etc do not hold accurately with your rarified gas. You will probably need to set your own viscosity, specific heat, density and conductivity using fortran subroutines USRVIS, USRSPH, USRDEN and USRCND. Look in a gas dynamics textbook for what values to use. So much for a simple problem....... Regards, Glenn

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