Secondary flow structure associated with interacting 3D bedforms

Barchan dunes are three-dimensional, crescent shaped bedforms found in both aeolian and subaqueous environments, including deserts, river beds, continental shelves, and even the craters of Mars. The evolution of and dynamics associated with these mobile bedforms involve a strong degree of coupling between sediment transport, morphological change, and flow, the last of which represents the weakest link in our current understanding of barchan morphodynamics. Their three-dimensional geometry presents experimental challenges for measuring the full flow field, particularly around the horns and in the leeside of the dunes. In this study we present measurements of the turbulent flow surrounding fixed barchan dune models in various configurations using particle-image velocimetry in a refractive-index-matching flume environment. The refractive-index-matching approach enables near-surface measurements, as well as access to the the whole flow field by rendering the solid models invisible. While experiments using solid models are unable to directly measure sediment transport, they allow us to focus solely on the flow physics and full resolution of the turbulent flow field in ways that are otherwise not possible in mobile bed experiments. The results presented here involve spectral analysis of flow in the wake of an isolated barchan, as well as in the interdune region of interacting barchans, using time-resolved cross-plane measurements.