Dust devils in the laboratory: Effect of surface roughness on vortex dynamics

摘要

Experiments simulating vortex interactions with rough surfaces were conducted at Earth ambient and Mars analog atmospheric conditions. Pressure profiles were obtained to assess the effect of nonerodible roughness elements on vortex structure at the surface. As roughness increased, vortex size increased and tangential velocity decreased. Particle threshold experimental results suggested that small increases in surface roughness enabled reduced threshold velocities to lift fine particles (<100pm) from the surface. This "optimal roughness" or the amount of roughness necessary for enhancing sediment transport from the surface, could allow weaker dust devils to lift more material from the surface than otherwise possible. Sediment flux was calculated for different sediment sizes and densities to determine how surface roughness affects the lifting potential by dust devils. Sediment fluxes were similar to previous studies with bulk averages ranging from 10 -5 to 1 kg m~-2 s~-1, but they could be subdivided based on roughness. The results showed that for the low roughness case (A 0.03), fluxes were at a maximum ranging from 10 -3 to 1 kg M~-2 s~-1 compared to two rougher surfaces (A 0.11 and 0.23). For the lowest roughness density the airflow around the elements is enhanced, whereas the higher roughness values showed more loss of energy to the surface, impeding sediment transport similar to boundary layer studies examining roughness effects on sediment transport.