Understanding Wind-Blown Sand and the Electrification of Granular Systems.

Abstract

This thesis presents a multi-faceted investigation into the physics of wind-blown sand, or ‘saltation.’ Using a combination of theory, laboratory experiments, and numerical modeling, I show that the frictional electrification of sand as it bounces along the soil surface enhances the concentration of saltating particles and causes them to travel closer to the surface. These findings seemingly resolve the discrepancy between the classical theory of saltation, which predicts that the height to which sand saltates increases sharply with wind speed, and measurements, which find that this height stays approximately constant with wind speed. This thesis also presents the first calculations of electric fields in Martian saltation. The results indicate that electric discharges do not occur in Martian saltation, and that the production of hydrogen peroxide and the dissociation of methane by electric fields are less significant than previously thought. Both these species are highly relevant to studies of past and present life on Mars. Particle electrification not only occurs during the wind-blown transport of dust and sand, but appears to be general to granular systems of chemically identical insulating particles. Measurements have generally reported that smaller particles in these systems charge negatively, while larger particles charge positively. I show that simple geometry leads to a net transfer of electrons from larger to smaller particles, and integrate this finding into the first quantitative charging model for a granular system of identical insulators. The results from this charging model are qualitatively and quantitatively consistent with measurements. Finally, this thesis presents a comprehensive numerical model of steady-state saltation. The model is the first to reproduce measurements of the wind shear velocity at the impact threshold (i.e., the lowest shear velocity for which saltation is possible) and of the aerodynamic roughness length in saltation. It also correctly predicts a wide range of other saltation processes, including profiles of the wind speed and particle mass flux. Since the model uses a minimum of empirical relations, it can be easily adapted to study saltation under a variety of physical conditions, such as saltation on other planets and saltating snow.