  Small scale jet-induced erosion experiments are useful for identifying the
scaling of erosion with respect to the various physical parameters (gravity,
grain size, gas velocity, gas density, grain density, etc.), and because they
provide a data set for benchmarking numerical flow codes. We have performed
experiments varying the physical parameters listed above (e.g., gravity was
varied in reduced gravity aircraft flights). In all these experiments, a
subsonic jet of gas impinges vertically on a bed of sand or lunar soil simulant
forming a localized scour hole beneath the jet. Videography captures the
erosion and scour hole formation processes, and analysis of these videos
post-test identifies the scaling of these processes. This has produced
important new insights into the physics of erosion. Based on these insights, we
have developed an erosion rate model that can be applied to generalized
situations, such as the erosion of soil beneath a horizontal gas flow on a
planetary surface. This is important to lunar exploration because the rate of
erosion beneath the rocket exhaust plume of a landing spacecraft will determine
the amount of sand-blasting damage that can be inflicted upon surrounding
hardware. Although the rocket exhaust plume at the exit of the nozzle is
supersonic, the boundary layer on the lunar surface where erosion occurs is
subsonic. The model has been benchmarked through comparison with the Apollo
landing videos, which show the blowing lunar soil, and computational fluid
dynamics simulations of those landings.
