Secondary Flows and Vortex Structure Associated With Isolated and Interacting Barchan Dunes

The three-dimensional, crescentic morphology of a barchan dune induces secondary flows and a complex vortex structure in its wake. In scenarios where barchans are in close proximity to each other, the flow modifications introduced by the wake of the upstream barchan are important for understanding the morphodynamics of the downstream barchan. The results herein detail the flow structure in a plane normal to the mean flow (cross-plane) through stereo particle image velocimetry measurements in a refractive-index-matching flow facility, utilizing solid, fixed-bed barchan models. Spatial distributions of streamwise-oriented swirling motions and Reynolds shear stress components reveal distinct flow regimes in the wake region of an isolated barchan: flow downstream of the horn tips and flow in the separated shear layer closer to the centerline. Streamwise rollers appear downstream of the horns, and measurements upstream demonstrate their origin on the stoss side of the dune in the form of a horseshoe vortex. Flow downstream of the separated shear layer in the wake embodies features consistent with that of hairpin vortices shed from the arched crestline of the barchan. These structures constitute the induction of secondary flows in the flow that, in the case of barchans in close proximity with a lateral offset, are preferentially amplified in accordance with local topography. Further analysis reveals the spatial scales and turbulent stresses associated with these structures, which are discussed in the context of larger fields of bedforms and the formation of protodunes.