Turbulence structure over idealized and natural barchan dune fields

LES simulation has been used to study inertia-dominated turbulent flows responding to aeolian sand dunes. The former is recovered from simulations initialized with a Reynolds-averaged flow, absent any small-scale features, which highlights the emergence of salient structures within the dune field roughness sublayer. 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 both applications, we report the emergence of mixing layer-like processes, 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 processes aloft 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 distance~to the wall or distance to the upflow dune, where the latter marks the beginning of the ``collapse'' stage.