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.

当平面沙面上方的湍流剪切流夹带沙粒时，会产生各种沙粒。河流砂形成两种主要的界面模式：中尺度沙丘和反沙丘，以及大型条形。测量证明，在侵蚀条件下，中尺度模式会变为大尺度模式或与大尺度模式共存。但是，究竟是什么驱动界面模式的转换以及如何进行转换，仍然难以捉摸。在这里，我们展示了-将流模型与谷物运输模型相结合，同时考虑了表面和悬浮的沙通量-模式的转换是由谷物运输和地形扰动之间的剪切驱动复杂反馈产生的。转换主要取决于Rouse数的大小和晶粒尺寸与未扰动的流动深度之比。