Formation Mechanism of Wind Ripples

Sand surfaces develop ripples and dunes in almost every known environment. Because of their ubiquity, ripples preserved in the geological record are invaluable to interpret past or remote climates, atmospheric conditions and fluvial regimes. In this context, understanding how ripples form is crucial to relate their size to environmental factors. According to conventional wisdom, decameter-scale dunes and decimeter-scale ripples on Earth emerge from clearly different processes related to wind motion and sand transport. However, recent experiments shattered this clean distinction by showing the emergence of an unexplained second type of wind ripple, somehow akin to water ripples and large ripples on Mars. The proposed investigation of this potentially ubiquitous new type of bedform can alter the interpretation of ripples in the field and the geological record, and shed light into some unexplored complexities of sediment transport. Broader impacts include the development of several public exhibits intended for K-12 students and educators, in collaboration with outreach experts at TAMU. The project will also provide research and educational opportunities for two early career principal investigators and undergraduate and graduate students. This research will investigate the emergence of these new “hydrodynamic” (also called “drag”) ripples on Earth using a combination of Computational Fluid Dynamics (CFD) and grain-scale transport simulations. The goal is to uncover the physical mechanisms controlling the origin and scaling of meso-scale bedforms (ripples) on Earth, and by extension, on water and other planetary bodies such as Mars, Titan and Venus. The three objectives are: validate and calibrate Hanratty’s hydrodynamic model, estimate the transport saturation length, and explain the transition from “impact” to “hydrodynamic” ripples. The central hypotheses are that there is a hidden sub-scale modulation of the aeolian transport layer that can explain the new observations and that hydrodynamic-driven instabilities become less relevant at a large enough grain size, for which only “impact” ripples are to be expected. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.