Experimental Study On The Stable Morphology And Self-attraction Effect Of Underwater Crescent Dunes

Barchan dunes are some kind of simple dune type which formed under the action of a unidirectional flow.Barchan dunes have attracted much attention because of their self-organization evolution and efficient migration.So far,the sand burial hazards caused by the migration movement of the barchan dune and the related desertification expansion have always been the key issues to be considered in the blowing sand control project.It is fact that natural aeolian barchan dunes usually present large temporal and spatial scales.Unfortunately,the small scaled barchan dune obtained in the wind tunnel experiment is difficult to reproduce its steady migration process.Therefore,in order to fully and effectively reveal the morphodynamic characteristics of the barchan dune,in this paper,the water medium is used to replace the wind medium,and the high-density aquatic environment is introduced as the downsized approach for the dune configuration.Under such constraint,the experimental studies on self-evolution and migration behavior of subaqueous dunes are carried out.The experiment is implemented on a closed circulating water tunnel.First,based on the controllable flow velocity regulation system,the experimental conditions of steady flow and unsteady sinusoidal flow velocities are set respectively.Then,the triangular,square and circular sand piles with the same mass are used as the initial sand source configurations.And the dynamic time sequence images of the dune evolution process under different flow and source conditions are recorded and obtained by optical camera at an upward angle.Finally,by developing an image extraction and post-processing algorithm based on Matlab code,the accurate comparison and analysis of the macroscopic profiles of dunes are realized.Results show that the trend of reproducing the crescent shape of the dune is shown in all sand piles.The triangular initial sand pile is found to be the first to stabilize to a crescent shape,whereas the square initial sand pile takes the longest time to form a stable crescent shape.This finding is essentially due to the shape effect of the triangle,which effectively reduces the amount of transverse moving sand particles.The dimensionless results of the shape profile of barchan dunes reproduced by the evolution of sand piles with different initial configurations are identical.This in essence proves that under the action of steady flow,the barchan dune presents the self-attraction effect of the crescent shape in the ‘‘streamwise-spanwise” dimension.Similar to the steady flow condition,under the unsteady flow condition,the crescent shape of the dune can also be reproduced in the sand piles of three different initial configurations,and the stable barchan dune also shows the self-attraction effect in the ‘‘streamwise-spanwise” dimension.However,in contrast to the results of the steady flow experiment,the order of the initial sand pile configuration from the fastest to the slowest for the first appearance of crescent shape under the unsteady flow condition,in turn,is triangle,square and circle.This may be attributed to the fact that the oscillatory characteristics of the sinusoidal flow velocity accelerate the evolution process of the sand piles with angular shapes.