Autodesk Simulation CFD provides a numerical solution of the Navier-Stokes (N-S) equations. The N-S equations assume that the fluid can be treated as a continuum. In some vacuum flows or very low pressure flows, the flow is no longer a continuum and individual fluid molecules must be considered. This assumption becomes inaccurate as the characteristic dimension of the flow path drops below 10 times the mean free path of the fluid.
|non-dimensional vacuum number|
|absolute viscosity at STP|
|L||characteristic length (hydraulic diameter)|
|< 0.014||The flow is a continuum and governed by the Navier-Stokes equations|
|0.014 < <1.0||The flow is slip flow where it slips along surfaces, but can still be approximated by the Navier-Stokes equations.|
|>1.0||The flow is no longer a continuum and cannot be represented by the Navier-Stokes equations.|