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

Abstract

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.

Spatial Scales of Turbulent Flow Structures Associated With Interacting Barchan Dunes

“…In collision C, at the “inside horn” of the UBD (i.e., the one that is now contacting the DBD toe), there is a very strong increase in the magnitude of positive θ structures, which are oriented more parallel to the stoss side of the DBD. As has been reported by Wang et al (), Wang and Anderson (), and Bristow et al (), this region where the horn of the UBD meets the toe of the DBD exhibits a “flow channeling” behavior, and the results shown here are consistent with this observation. The orientation of flow structures is consistent with flow being very strongly influenced by local topography, rather than the shear layer produced around the horn of the UBD that would induce the opposite orientation.…”

“…Immediately downstream, these large length scale streaks form the outside edge of the shear layer that is located on both sides closer to the centerline. A narrow strip of smaller turbulence scales is visible along the centerline, extending well beyond the point of mean reattachment, which occurs along the centerline at the bed at x / H = 5.0 (Bristow et al, ). Such a structural feature in the flow is likely the result of the convergence and interaction between eddies with opposite rotation, as indicated by a spanwise profile of swirling strength in the wake (Figure d), resulting in a breakup of coherence and thus a reduction in scale.…”

“…To address this issue, length scales near the bed in the wake of the isolated barchan (in the x ‐ y plane at the centerline) are calculated by forming a conditional ensemble of snapshots of the flow, conditioned on flow events featuring increasingly intense positive streamwise velocity fluctuations. For a given instantaneous snapshot of the flow, the wake region at an elevation of y / H = 0.15 (chosen as the same elevation used by Wiggs & Weaver, , and Bristow et al, ) from the x ‐ y centerline plane is searched for positive streamwise velocity fluctuations that satisfy ( + ) u > Mu th , where u th is a threshold value equal to the rms streamwise velocity in the flow far upstream of the barchan at the same elevation above the bed and M is a multiplication factor ranging from 0 to 3. At the streamwise position where this threshold is met, the flow field around this reference position within x ref ± 4 H is extracted.…”

“…As far as subaqueous barchans are concerned, field studies involving flow measurements have not been reported in the literature, although several laboratory investigations have been undertaken, including that of Charru and Franklin (), who focused on the stoss side of an isolated self‐formed subaqueous barchan, and the offset, fixed‐bed interacting barchans considered by Bristow et al (). This limited study is in contrast to the wide body of literature that has examined 2‐D dunes (e.g., Bennett & Best, ; Best, ; Kwoll et al, ; Livingstone et al, ; Nelson et al, ; Schindler & Robert, ) and generic 3‐D dunes (e.g., Maddux, Nelson, et al, ; Maddux, McLean, et al, ; Venditti, ) using water channels.…”

“…Green lines in (b) and (e) mark the streamwise‐wall‐normal and streamwise‐spanwise measurement plane positions, which are the same for all configurations, while in (f) the slopes of the stoss and lee sides are indicated along the dune centerline. Adapted from Bristow et al ().…”