Investigating the interactions between an atmosphere and an ejecta curtain 2. Numerical experiments

The locus of ejecta excavated during an impact generates a debris curtain that expands outward. In an atmosphere this advancing curtain acts like a semipermeable barrier that displaces the surrounding gas. The generated flow separates near the top of the curtain to form a vortex ring whose strong winds entrain, transport, and deposit fine-grained ejecta, affecting the morphology of distal ejecta deposited on planets with atmospheres. We have investigated how the curtain width and velocity, particle concentration, size distribution and velocity parallel to the curtain, and the density, viscosity, and compressibility of the surrounding atmosphere controls the flow strength of these winds. Wind tunnel tests (Part I [Barnouin-Jha et al., this issue]) show that for an ejecta-like porous plate, the hydraulic resistance, a measure of energy losses for one-dimensional porous flow, governs the position along the curtain where it becomes effectively impermeable. Combined with suitable cratering models and published hydraulic resistance data, this information allows estimating the flow strength or circulation generated by an advancing curtain. The present study assesses the influence of atmospheric compressibility and particle motion parallel to the curtain surface on the curtain's circulation in order to improve these estimates. Numerical experiments indicate that atmospheric compressibility has little effect on the circulation at Mach number below 0.5, consistent with analytical solutions. Analytical solutions show, however, that this flow circulation should increase significantly at higher Mach numbers. The numerical experiments also show that individual ejecta traveling parallel to the surface of the curtain enhance the induced circulation by 9% to 33%.