A gas with such low density that molecular collisions are rare, invalidating continuum assumptions.A rarefied gas is a gas with such low density that the average distance between molecules is large compared to the typical scale of the system, making molecular collisions rare. In this regime, the continuum assumption of fluid dynamics—which treats the gas as a smooth, continuous medium—breaks down. Instead, the gas behaves as a collection of individual particles, and phenomena like slip flow, thermal transpiration, and non-equilibrium velocity distributions become important.
The key parameter is the Knudsen number, the ratio of the molecular mean free path to a characteristic length scale. When this number is high, molecules travel ballistically between surfaces, and their interactions are dominated by collisions with boundaries rather than with each other. This requires kinetic theory or direct simulation Monte Carlo methods to model accurately, rather than the Navier-Stokes equations.
Rarefied gas dynamics is essential for designing vacuum systems, micro-electromechanical devices (MEMS), high-altitude aircraft and spacecraft re-entry, and for understanding planetary atmospheres at high altitudes.
In a rarefied gas, temperature can be different in different directions, a concept known as 'thermal non-equilibrium,' which is impossible in a dense gas.
