Interfacial instability of sand patterns induced by turbulent shear flow

Abstract

When a turbulent shear flow above a plane sand surface entrains sand grains, it generates a variety of sand patterns. Fluvial sand forms two major interfacial patterns: meso-scale dunes and antidunes, and large-scale bars. Measurements have evidenced that under erosive conditions, meso-scale patterns either change to or coexist with large-scale patterns. However, it remains elusive what exactly drives the switching of interfacial patterns and how the switching occurs. Here, we show—combing a flow model with a grain transport model, allowing for both the surface and suspended sand fluxes—that the switching of patterns emerges from the shear-driven complex feedback between grain transport and topographic perturbations. The switching predominantly depends on the magnitudes of the Rouse number and the grain size to undisturbed flow depth ratio. The model offers quantitative predictions of the maximum amplification of sand patterns and unveils a new attraction–repulsion phenomenon.