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.

The spatial coherence of turbulent flow structures throughout the flow field associated with a collision between a smaller upstream barchan laterally offset from a larger downstream barchan is investigated using inhomogeneous, two‐point correlation coefficients of fluctuating streamwise velocity, from which the distribution, size, and orientation of the large‐scale motions in the flow are analyzed. Measurements were made with fixed‐bed models in a refractive‐index‐matched flume environment allowing uninhibited optical access where the flow field is captured using particle‐image velocimetry in both streamwise‐wall‐normal and streamwise‐spanwise planes. The shear layer of a barchan produces flow structures of smaller length scale, yet still on the order of the barchan height, and stronger positive streamwise fluctuations near the bed as compared to the incoming boundary layer, and these effects prevail far downstream. Analysis of the orientations of the flow structures suggests secondary flow motions induced by the barchan horns, and interdune flow modification through collision stages indicates changing flow interaction regimes with decreasing interdune separation. The combination of enhanced positive streamwise fluctuations near the bed and significant spatial coherence indicates that for a field‐scale barchan of sufficient size, the coherence of flow structures produced in its wake is of comparable scale to the characteristic drag length associated with aeolian transport. As length scales nominally scale with barchan height, the morphodynamics of collisions between barchans of disparate size can be partially explained through this paradigm of flow structure coherence.

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.

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