Knudsen number (Relationship to Mach and Reynolds numbers in gases)

Description

The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of the body in a fluid. (The mean free path is the average distance travelled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions), which modify its direction or energy or other particle properties.) Mach number is representing the ratio of speed of an object moving through a fluid and the local speed of sound. The Reynolds number is defined as the ratio of inertial forces to viscous forces. There is a relation between Knudsen number and Mach and Reynolds numbers in gases.

Related formulas

Variables

KnKnudsen number (dimensionless)
MaMach number (dimensionless)
ReReynolds number (dimensionless)
γThe ratio of specific heats (dimensionless)
πpi