Breakdown of continuum flow in freejets and rocket plumes

Abstract

A previously obtained empirical criterion for the breakdown of continuum flow in one-dimensional expansions has been applied to the two-dimensional Prandtl-Meyer expansion around a corner and to typical axially symmetric freejet and nozzle plume flows into a vacuum. The nonequilibrium effects in the Prandtl-Meyer flow are found to be confined to the gas that enters the expansion within about twenty mean free paths from the corner. In the axially symmetric flows, all the flow eventually becomes noncontinuum, but the results indicate that the plume backflow region is primarily determined by the flow in the immediate vicinity of the nozzle lip. The direct simulation Monte-Carlo method is used to obtain solutions for this critical flow region. This confirms the usefulness of the empirical breakdown criterion and verifies that rocket exhaust plume backflow predictions must be based on a discrete particle analysis of the flow in the lip region.