FLOW DYNAMICS ASSOCIATED WITH INTERACTING BARCHAN DUNES IN TANDEM COLLISION AND EJECTION ARRANGEMENTS

Barchan dunes are naturally occurring three-dimensional topographic features that have been observed on the surface of several planets. They occur both in aeolian environments characterized by sandy deserts (e.g. White Sands National Monument in New Mexico) as well as in subaqueous environments. Barchan dunes form in regions with weak variability in flow direction and limited sediment supply wherein the flow mobilizes the sediments and produces bedforms with crescent shaped quasi-symmetrical ridges and horns directed downwind, known as barchanoids. Barchan dunes typically form in fields having a broad distribution in dune size, with each bedform within a field characterized by a typical migration rate that is inversely proportional to the bedform size. Different migration rates among dunes result in variable bedform spacing and eventually dynamic bedform−bedform interactions (e.g. collision, merging, splitting). These morphodynamic processes are controlled by complex feedback mechanisms mutually linking three key elements: fluid flow, sediment transport and bed morphology. The aim of this work is to contribute to the understanding of the fluid-flow mechanisms responsible for the formation, migration and interaction of these dunes. To this end, we study the three-dimensional flow generated by the interactions between fixed barchan-dune models arranged in tandem in collision and ejection scenarios via experiments in an optically-accessible flow environment using planar particle-image velocimetry (PIV) measurements of the flow field. These measurements are complemented by targeted large-eddy simulations (LES) meant to provide a three-dimensional view of the flow processes for these fixed dune arrangements.