Collisionless Expansion of Gas Clouds in the Presence of an Ambient Gas

The collisionless expansions of spherical and cylindrical gas clouds into an ambient gas are investigated. General expressions for the higher moments are derived and evaluated. The solutions exhibit effects not apparent in previous work on collisionless expansions bounded by vacuum. Mass flow, temperature and velocity exhibit propagating maxima which decay in amplitude and diffuse with time. Time constants for the duration of the expansions are obtained as a rough criterion for possibility of shock formation. Comparisons are made with the Euler solution and a short time collisional solution obtained by numerical integration of the Boltzmann equation with a model collision term.