Large-eddy simulation of turbulent flow over spanwise-offset barchan dunes: interdune roller sustained by vortex stretching.

The feedback between fluid-flow processes and barchan dune geometry is a key determinant in the evolution of dune fields. Large-eddy simulation was used to model turbulent flow over a series of static barchan dunes. The dune configurations captured realistic stages of a so-called "offset interaction", wherein a relatively small dune is placed upflow of a relatively larger dune, thereby guaranteeing interaction since the former migrates faster than the latter. We conditionally sampled the flow based on events low in probability but high in magnitude; this is relevant to erosion and sediment transport since models indicate a nonlinear scaling between aero-/hydro-dynamic surface stress and sediment flux. Conditional sampling revealed a persistent roller in the space between the upflow and downflow dune. We show that the rotational sign of the roller drives scour from the large dune, and facilitates dune interaction. Terms in the Reynolds-averaged streamwise vorticity transport showed that the roller is primarily sustained by stretching in the interdune space. With this, we show isocontours of local (differential) helicity, which confirms that the roller is persistently scouring sediment from the downflow dune.