On aeolian transport: grain-scale interactions, dynamical mechanisms and scaling laws.
Résumé
Aeolian processes involve the wind action on a sedimentary substrate, namely erosion, sand transport and deposition. They are responsible for the emergence of aeolian dunes and ripples but also erosive structures like yardangs. In this review, we discuss the physics of aeolian sediment transport from a physical point of view. Relevant time and length scales associated to turbulent wind fluctuations are summarized using aerodynamic theory. At the microscopic scale, the different forces acting on the grains are detailed. We then introduce the concepts -- e.g. saturated flux, saturation length -- and the relevant framework for the development of a continuum quantitative description of transport. Static and dynamical entrainment thresholds are modeled and discussed. Steady transport is investigated in two asymptotic regimes: close to threshold and far above it. In both cases, a simple picture, taking into account the negative feedback of particles on the wind flow, is analytically drawn and compared to experimental and numerical data. The low wind velocity regime corresponds to the model proposed by Ungar and Haff (1987) and the high wind velocity regime is elaborated from initial ideas of Bagnold (1941). Transport transient is also studied in detail, and scaling laws for the saturation length are proposed. Finally, some open issues for future research are outlined in the conclusion.