Turbulent Flow Structure Associated with Interacting, Laterally Offset Barchan Dunes

Barchan dunes are three-dimensional, crescent-shaped bedforms which, as they migrate in the direction of the overlying wind or water flow, interact with their neighboring bedforms. Smaller, faster migrating barchans will overtake and collide with larger, slower moving ones, resulting in complex morphological changes, enhanced sediment erosion, modified flow dynamics. The primary aim of this study is to elucidate the associated turbulent flow structure which is hypothesized to drive the morphodynamics of such barchan--barchan interactions.

Flow measurements were made using particle-image velocimetry (PIV) over configurations of solid barchan models mimicking stages of a laterally offset collision process, in addition to a baseline, isolated case. To obtain full optical access, experiments were conducted in refractive index matching (RIM) flow facilities.

First, low frame-rate PIV data was collected in streamwise--wall-normal and streamwise--spanwise measurement planes in order to identify the mean flow structure, spatial distributions of single-point turbulence statistics, and flow coherence through two-point correlations. Analysis of this dataset revealed a significant modulation of the flow over the downstream barchan by the upstream barchan in terms of enhanced near-bed turbulent stresses and modified flow separation. Distinct flow regimes are shown in the wake of the isolated barchan, which are modified in the interdune region for collision scenarios. The associated flow lengthscales and near-bed turbulence statistics aid in explaining sediment erosion potential and collision morphodynamics.

A combination of low and high frame-rate cross-plane stereo-PIV measurements were then made in a large-scale RIM flow facility to further elucidate the dynamics and three-dimensionality of characteristic flow structures induced in the wake of the barchan. These measurements showed an organization of secondary flows linked to the unique morphology of the barchan dune. Analysis of the turbulent stresses, vortex identification, and spectral analysis further suggest that distinct flow regimes in the wake regions are linked to a combination of horseshoe and hairpin-like structures that induce these secondary flows.

The results and discussion herein provide a framework for interpreting observed morphologies of interacting barchan dunes, and build a foundation for further study of potentially important field-scale dynamics, as well as extensions to other three-dimensional bedforms.