Turbulence Coherence Within Canonical and Realistic Aeolian Dune-Field Roughness Sublayers

Abstract

Large-eddy simulation has been used to study the formation and spatial nature of inertia-dominated turbulent flows responding to aeolian sand dunes. The former is recovered from simulations initialized with a Reynolds-averaged flow, without any small-scale features, which highlights the emergence of salient structures within the dune-field roughness sublayer (RSL). The latter is based upon computation of integral lengths. In the interest of generality, these exercises are based upon flow over canonical dune geometries—which serve as a comparative benchmark—and flow over a section of the White Sands National Monument aeolian dune field in southern New Mexico. These cases, thus, capture a vast range of complexity. In both applications, we report the emergence of mixing-layer-like processes—as per results for other canopy flows—although the distinct geometric nature of the dunes shows the prevalence of a persistent interdune roller, which is aligned most closely with the streamwise direction. In order to demonstrate underlying similarities in the processes occuring above idealized and natural dune fields, we normalize the integral lengths by characteristic length scales: vorticity thickness, attached-eddy-hypothesis mixing length, and dissipation length. This exercise reveals a distinct growth and collapse pattern that is robust across all considered dune arrangements. Herein, ‘growth’ refers to the stage of downflow thickening of vortices produced via vortex shedding off the upflow dune; growth is regulated by the lesser of the distance to the wall or distance to the upflow dune, where the latter marks the beginning of the ‘collapse’ stage. Both are compliant with the notion of wall-attached eddies. In the RSL, we demonstrate that the integral lengths exhibit an optimal collapse when normalized by vorticity thickness, while inertial layer scaling is attained as close as one dune height above the top of the dune canopy. These results help to establish dune-field RSL dynamics within the broader context of canopy turbulence, which is important given the relatively greater efforts devoted to flows over vegetative canopies and urban environments.