Turbulent Flow Structure Associated With Collision Between Laterally Offset, Fixed-Bed Barchan Dunes

Barchan dunes are three-dimensional, crescent-shaped bedforms found in regions of unidirectional flow and limited sediment supply, and while most commonly associated with aeolian environments, they have recently been observed in subaqueous domains and on the surfaces of Mars and Titan. As barchans migrate in the direction of the flow, they interact with their neighbors, typically by way of collision. The morphodynamics of such collision processes are complex, where the role of the turbulent flow structure is strongly coupled to that of sediment transport and morphological change. Here we study the flow structure in a decoupled manner through measurements of turbulent flow over fixed-bed barchan models in a series of configurations mimicking the early stages of a laterally offset collision between two dunes. Particle image velocimetry is used to measure the flow in a refractive-index matched flume that enables uninhibited access to the flow field to study the role that the upstream barchan plays in modifying the turbulent structure over the downstream one. Flow sheltering by the upstream barchan results in a momentum deficit in its wake that is also marked by heightened levels of turbulence over the downstream barchan. Additionally, flow asymmetry induced by the lateral offset between barchans drives a flow channeling effect around the horn of the downstream barchan. Decomposition of Reynolds shear stresses in the interdune space via quadrant analysis reveals that near-bed turbulent fluctuations capable of mobilizing sediment are particularly enhanced at mean flow reattachment where vortices within the shear layer impact the bed.