Spatial Scales of Turbulent Flow Structures Associated With Interacting Barchan Dunes

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.