CFD Online Discussion Forums

CFD Online Discussion Forums (
-   Main CFD Forum (
-   -   pressure variations in low vacuum (10e-3 to 1 bar) (

cecile baril November 26, 2001 13:08

pressure variations in low vacuum (10e-3 to 1 bar)

I have to lead a project about low vacuum, and I don't really know what are the differences between the equations in case of high pressures and the equations in case of vacuum. Do you have informations about that and do you know the way to calculate the variation of pressure along a 40mm diameter pipe ? My study concerns only air. I don't know the pump flow and the applications ?

Thank you for your help,


P.Fonteijn November 27, 2001 03:50

Re: pressure variations in low vacuum (10e-3 to 1
At high pressure the gas molecules will interact heavily and you will have to use rho=p*M/(z*R*T) with z=z(T,p,.....whatsoever).

The lower the pressure, the easier it will be. Then you can easily treat air as an ideal gas, so rho=p*M/(R*T).

As far as I know all continuum and NS-equations are applicable.

Good luck, Pascale.

Johan Larsson November 27, 2001 12:12

Re: pressure variations in low vacuum (10e-3 to 1
The Navier-Stokes and continuity equations are derived by using the continuum approximation, which means that the mean free path of the molecules has to be much, much shorter than any physically relevant dimension.

I'm not very familiar with the area of vacuum, but I would assume that your pressure (0.001 bar) would be high enough for the Navier-Stokes equations to be valid. You should, however, check this in any basic fluid mechanics book!


All times are GMT -4. The time now is 19:16.