Validity of NS-eq at low pressures
 

Hi all,

I would like to know if anyone have experience in CFD calcluations at very low pressures, say 0.01 Pa. Are the Navier-Stokes equations still valid at such low pressures? Is there a limit anyway for the NS-eqs.?

Pascale

Re: Validity of NS-eq at low pressures
 

I think your problem is not well raised. And I think you should change your pressure into adimensional conditions. And at that time, it is still much lower than other parameters, the NS equations may not to be correct any longer. On another side, the pressure is too low to consider your fluid to be continuum, the NS equation is no longer accepted.

Re: Validity of NS-eq at low pressures
 

You must look at the Knudsen number (Kn), which is the ratio of the molecular mean free path to the characteristic length scale of the flow. If Kn<<1 then the continuum models are valid. If Kn=O(1), then it is not.