Large-eddy simulation of turbulent flow over spanwise-offset barchan dunes: Interdune vortex stretching drives asymmetric erosion

Abstract

The coupling between turbulent flow physics and barchan dune geometry is important to dune migration, morphology of individual dunes, and the morphodynamics of merging and separating proximal dunes. Large-eddy simulation was used to model turbulent, inertial-dominated flow over a series of static barchan dune configurations. The dune configurations were carefully designed to capture realistic stages of a so-called ``offset interaction,'' wherein a small dune is placed upflow of a relatively larger dune, thereby guaranteeing interaction since the former migrates faster than the latter. Moreover, as interaction proceeds, the morphology of the small dune is mostly preserved, while the large dune undergoes dramatic transformation with greater erosion downflow of the interdune space. Simulations reveal that the wake centerline---determined here as the spanwise location at which the momentum deficit associated with each dune exhibits a minimum---veers due to dune geometry. Visualization of vortex identifiers reveals that hairpin vortices are produced via separation across the crestline of dunes, and these hairpins are advected downflow by the prevailing, background flow. The legs of hairpins emanating from the upflow dune contain streamwise vorticity of opposite sign, wherein the hairpin leg within the interdune space exhibits positive streamwise vorticity. This positive streamwise vorticity is supplied to the interdune space, where flow channeling induces acceleration of streamwise velocity. An assessment of right-hand side terms of the Reynolds-averaged streamwise vorticity transport equation confirm, indeed, that vortex stretching is the dominant contributor to sustenance of streamwise vorticity. With this, we can conclude that asymmetry of the large, downflow dune is a consequence of scour due to the interdune roller, and scouring intensifies as the spacing between dunes decreases. A structural model outlining this process is presented.