Rarefied gas dynamics

Rarefied gas dynamics is a branch of fluid mechanics where the continuum assumption is no longer accurate, as a characteristic length scale in the gas (e.g. radius of a body moving in a gas, radius of a tube conducting a gas, etc.) becomes comparable to the mean free path of gaseous particles. Consequently, the gas cannot be described as a continuum. Instead, the gas should be described by its microstate determined by the velocity and position of each particle. Since it would be an impossible computational task in storing the very large volumes of information involved in tracking the behavior of every single particle, the statistical or kinetic theory of gases must be used. This theory consists in obtaining the macroscopic state of the gas described by quantities such as density, bulk velocity, temperature, pressure tensor, and heat flux from the microstate of the gas. To describe non-equilibrium phenomena in rarefied gases, the Boltzmann transport equation must be used, which is the appropriate mathematical tool for this purpose.