# Compressible or Incompressible?

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 November 14, 2004, 22:15 Compressible or Incompressible? #1 Samy Guest   Posts: n/a I have a quick question. If I choose air 25deg,1atm in the pre 5.7, will the solver treat the air as incompressible flow? Thank you, Samy.

 November 15, 2004, 04:11 Re: Compressible or Incompressible? #2 test Guest   Posts: n/a Hi, well air at 25 C is a constant property liquid so the solver will not treat as incompressible. You will have to use Air as an ideal gas which will CALCULATE THE DENSITY BASED ON IDEAL GAS LAW. iF YOU HAVE UR OWN compressibility equation you will have to create your own expression for density which can be based on pressure or temperature or both of them

 November 15, 2004, 09:21 Re: Compressible or Incompressible? #3 Samy Guest   Posts: n/a Hi test, Thank you for your help but I'm not quite get what you mean in the first sentence.. How come the solver doesn't treat air at 25C as "incompressible" when it hass a constant fluid property? I thought incompressible flow has a rather constant fluid property, which changes very little when pressure & temp changes. Thanks,Samy

 November 15, 2004, 12:36 Re: Compressible or Incompressible? #4 SAM Guest   Posts: n/a ya u r right but solver run on equations, be defualt most of the softwares have constant value for density but if u want to change that property then u have to mention density calucation based on ideal gas law. moreover fluid property also depends on geometry and flow conditions. may be there is huge tempreture increase and pressure jump (shock wave) in that case incompresible assumption may be wrong.

 November 15, 2004, 17:42 Re: Compressible or Incompressible? #5 Glenn Horrocks Guest   Posts: n/a Hi, To use compressible air (as opposed to incompressible air) as the working fluid, you have to select Air ideal gas as the fluid, and on the heat transfer model tab you have to select Total energy. The other two selections would give you an incompressible fluid with properties evaluated at the given temperature. Glenn Horrocks

 November 18, 2004, 13:16 Re: Compressible or Incompressible? #6 Tony Guest   Posts: n/a please correct me if I am wrong. For premixed combustion in which fuel/air mixture is injected through inlet, when I create the fuel/air mixture, I set it as ideal mixture, the solver will treat each component in the mixture as ideal gas. But what about the combusted gas in the domain? Thanks.

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