Prandtl-Meyer flow of a finite Knudsen number gas

The continuum description of a gas expanding around a sharp corner to a vacuum becomes inappropriate when the distance from the corner is small. An existing criterion for the breakdown of continuum flow is used to predict the nature of the process in the classical Prandtl-Meyer expansion flow. These predictions are checked by a numerical analysis of the flow using the direct simulation Monte Carlo method. This verifies that the affected region is largely confined to the gas that is initially within about twenty mean free paths from the corner. It also shows that there is an additional breakdown region near the start of the expansion and extending upstream of it.