Self-stabilization of barchan dune chasing
Description
Barchan dunes are often found in a variety of environments. They exhibit group-like self-stabilizing characteristics when interacting with each other. Under the premise of local similarity, water tunnel experiments have been widely used to study the fascinating dune interactions, especially contact interactions, i.e., collisions; however, non-contact interactions (i.e., chasing) and the fluid mechanisms behind them have not been studied sufficiently. To bridge this gap, we performed water tunnel experiments combined with numerical simulations to demonstrate the flow structure around two chasing dunes. Our results suggest that the distance between two dunes may become stable due to a combination of three factors: repulsion effect between the two dunes, which widens the distance, and impediment to downstream dune movement, embracing effect which shrinks the distance, and the difference in dune size, whose specific effect depends on the relative position of the two dunes. The first two factors are achieved by two special vortex structures. Based on this model, we propose a scaling law to describe the equilibrium of the dune chasing and a governing equation to explain the breakdown of such equilibrium, and they are both experimentally verified. Our model provides a framework to explain the self-stabilizing mechanisms of barchan dune swarms. The supporting information contains eight movies and their accompanying images as examples of our experiments, as well as the raw data of the dune distance over time of all the tests.