Investigating the interactions between an atmosphere and an ejecta curtain2. Numerical experiments

The locus of ejecta excavated during an impact generates a debris curtain t hat expands outward. In an atmosphere this advancing curtain acts like a se mipermeable barrier that displaces the surrounding gas. The generated flow separates near the top of the curtain to form a vortex ring whose strong wi nds entrain, transport, and deposit fine-grained ejecta, affecting the morp hology of distal ejecta deposited on planets with atmospheres. We have inve stigated how the curtain width and velocity, particle concentration, size d istribution and velocity parallel to the curtain, and the density, viscosit y, and compressibility of the surrounding atmosphere controls the flow stre ngth of these winds. Wind tunnel tests (Part I [Barnouin-Jha et al., this i ssue]) show that for an ejecta-like porous plate, the hydraulic resistance, a measure of energy losses for one-dimensional porous flow, governs the po sition along the curtain where it becomes effectively impermeable. Combined with suitable cratering models and published hydraulic resistance data, th is information allows estimating the flow strength or circulation generated by an advancing curtain. The present study assesses the influence of atmos pheric 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 solut ions. Analytical solutions show, however, that this flow circulation should increase significantly at higher Mach numbers. The numerical experiments a lso show that individual ejecta traveling parallel to the surface of the cu rtain enhance the induced circulation by 9% to 33%.