Rigorous, Practical Method for Predicting Plume Backflow and Surface Impingement

Spacecraft surface contamination can result from nozzle-exhaust molecules, droplets, and/or condensate-clusters which expand and scatter to fill the space volume at large angles from the nozzle thrust axis. Only a small fraction of the exhaust products expands to large angles; however, the amount is sufficient to influence the design and performance of spacecraft sensor platforms which are subject to contamination effects. The large-angle plume mass/species distribution functions depend on 1) the interacting effects of pressure/shear forces and diffusion/rarefaction effects during the rapid angular expansion at the nozzle lip and 2) the nozzle boundary layer characteristics upstream of the lip expansion. Current prediction methods include 1) method-of-characteristics for continuum regions and 2) direct simulation Monte Carlo (DSMC) techniques for transitional regions. Composite predictions are cumbersome to implement; extremely high grid resolutions are required to preclude numerical diffusion across steep gradients. A new formulation is proposed which uses an extended form of the Navier-Stokes equations in flow-conformal curvilinear streamtube coordinates (to minimize numerical diffusion effects) with additional source/sink terms (to account for flow rarefaction effects). The formulation provides a unified continuous prediction capability which bridges from the continuum region of the nozzle interior to the rarefied region of the plume backflow.