Whole-field velocity measurements of flow around colliding barchan dunes

Abstract:

Barchan dunes are crescentic bedforms located in environments with unidirectional flow and limited sediment supply, including deserts, river beds, the continental shelves and the craters of Mars. Barchans are commonly observed in fields rather than in isolation, with the evolution of, and interactions between, bedforms being highly dynamic, involving feedback mechanisms between the fluid flow, morphological change and sediment transport. A series of experiments were undertaken to discretely simulate the collision of a smaller barchan with a larger, downstream one using fixed bedform models, with each experiment representing a successive snapshot in the dune collision process. These experiments thus capture the turbulent flow over fixed-bed morphologies that correlate with rapid morphological change and high rates of sediment transport using time-resolved particle image velocimetry (PIV) in the wall-parallel plane. The use of a Refractive Index Matching (RIM) flow facility allows for the light to pass through the model, capturing areas which are otherwise obscured, such as around the horns of the dune, the sheltered region behind the crest, and areas in which the bedforms are deformed during the collision. This paper will present the results of Dynamic Mode Decomposition that has been used to identify the most dominant modes contributing to flow dynamics in each collision stage.