Numerical and experimental study of flow over stages of an offset merger dune interaction

Highlights

• Demonstration of wake veering in roughness sublayer of turbulent flow over sand dunes.

• Demonstrated close agreement between numerical and matching experiment case.

• First- and second-order statistics show channeling between streamwise-staggered dunes.

Abstract

Results of a coordinated research effort on unidirectional turbulent flows over canonical barchan dunes at high Reynolds number are presented. Large-eddy simulations (LES) and experiments are conducted under inertial-dominated flow conditions, thus capturing dynamics essential to fully understanding processes responsible for observed patterns of asymmetric erosion and migration. A series of dune field topographies have been considered wherein a small “impactor” dune is positioned at a series of positions upflow of a large “parent” dune, from a spanwise-offset position; this interaction constitutes a so-called “offset interaction” Kocurek and Ewing [14]. The small dune is geometrically similar, but one-eighth the volume of the large dune. Since migration rate is inversely proportional to volume, the prescribed volumetric ratio ensures that the synthetic topographies replicate instantaneous configurations exhibited during actual dune interactions in a laboratory or natural setting. In this sense, the static dune configurations provide a means to understand flow processes responsible for patterns of erosion and deposition that induce interactions. Experimental measurement and LES are both used to study these configurations, with strong agreement reported between resultant datasets. We report that flow channeling in the interdune space between the upflow and downflow dune induces a mean flow heterogeneity – termed “wake veering” – in which the location of maximum momentum deficit in the dune wake is spanwise displaced. We also report elevated turbulent stresses in the channeling region. These results are used to explain the mechanisms responsible for simultaneous merging and ejection between the small and large dunes. Finally, spatial distributions of surface stress from LES have been used to identify locations of elevated erosion and, therefore, to predict bedform migration patterns. Results show that locations of minimal erosion – whether associated with upflow sheltering or with vanishing spatial gradients of dune height – constitute spatial “junctions” of coalescing, proximal dunes. In the absence of upwind sheltering, or in locations with the least upwind sheltering, turbulent mixing facilitates downward fluxes of high momentum fluid. This augments downwind dune erosion, cumulatively accelerating the migration of downwind dunes.