Expansion of a gas cloud by the discrete velocity method

A discrete velocity model is presented to reduce the Boltzmann equation into a set of equations which are used to solve the unsteady expansion of a rarefied (Kn ranging from 0.1 to infinity) gas cloud bounded by a vacuum. Good agreement is observed among results derived by the present method and those of the free molecular flow method. The discrete velocity method is to divide the moving directions of the molecules into finite (here, two) groups, then a discrete velocity model is formed based on this division. In this way the effect of molecular collisions is taken into account.