Evolution of barchan dune interactions investigated by a downscaled water tunnel experiment: the temporal characteristics and a soliton-like behavior
The downscaled water tunnel experiment can achieve local similarity in sand dune morphology with the field results while avoiding the large spatiotemporal scales in field observations. A classic example is the solitary-wave-like behavior of dunes. A small dune merges in the upstream face of a large dune and pushes out a dune of similar size from the leeside of the large dune, which appears as if this small dune goes straight through the large one under monochromatic conditions. For this phenomenon, the existence in water tunnel experiments has been confirmed before the satellite image evidence was established. Aside from the solitary-wave-like behavior, researchers have discovered several different dune interaction patterns through water tunnel experiments, which are affected by various factors, such as flow rate and initial mass ratio of the two dunes. Assumptions show that when two dunes are close in size, they keep moving at the same celerity because the size of a dune is inversely proportional to its speed. In this paper, a new pattern emerges unexpectedly because of the flow structure between the two dunes. Specifically, the tip of the downstream dune is captured by the reversing flow produced by the upstream dune and disengages from the body of downstream one under the conditions of suitable mass ratio and spacing. Together with the upstream dune, this tip dune displays an amazing soliton-like behavior. Different from the abovementioned solitary-wave-like behavior, this soliton realizes the mutual crossing of dunes and the maintenance of self-mass at the same time within a certain space-time range. The reasons for its occurrence and the conditions affecting its survival are analyzed.