Analysis Of The Evolution Process Of Underwater Barchan Dunes Under Unsteady Flow Condition

Barchan dunes can be found in the deserts and seabed on earth on the other celestial body surfaces like Mars and Venus.Barchan dunes can bring land eroding,structures damage,and a direct threat to human life and health.The study of barchan dunes’ morphological and kinematic mechanism is significant to fix the barchan dunes and reduce the natural disasters caused by the dunes’ transport.In this paper,two aspects of the research content have been carried out.One is to establish the identification method of barchan dunes.The other one is to investigate the morphology and kinematics of the subaqueous barchan dune during its evolution under unsteady incoming stream conditions.In this paper,the method of dividing the curve is adopted to realize the extraction of the curve and extreme points of the avalanche on the outer contour of the barchan dune,as well as the accurate extraction of all the characteristic points of the barchan dune contour.The identification model and method of barchan dunes are established,and the corresponding standard barchan dunes images are calculated and drawn according to the identified characteristic points of the dunes.A series of processing is performed on the edge contours of the two images to obtain an equal number of polar coordinate discrete curves.Substitute into the discrete Frechet similarity calculation formula to analyze and determine whether a dune is a barchan dune.The accurate identification of the barchan dunes is realized with MATLAB software programming.By setting the inverter signal as a sine signal the pump was made to run with velocity changing as a sine curve.The result of the water flows velocity measurement shows that the stream velocity also changed sinusoidally.while changing the period and amplitude of the sine curve of the inverted frequency,the ratio of width to length of the dune would fluctuate greatly in the early stage,and then fluctuate stably around the "equilibrium size".The effect of the amplitude variation on the ratio of the height to length is large than that of the frequency variation.Whether amplitude or the period of the inverted frequency changed,the changing trend of the ratio of the dune area to the area of the initial sand pile is transient at first and then continuously decreases during the whole evolution process.Notably,the ratio change shows a‘ladder’ decline under the conditions of the amplitude varying.By analyzing the length variation of the left and right horns of the barchan dunes under the conditions with different period and amplitude parameters,it can be found that the length variations of the left and right horns alternately increase and shorten,which may be caused by the periodicity of the incoming water flow velocity.Lastly,it can be found by the kinematic analysis that the amplitude and period of the dune migration displacement and velocity are similar to the change of water flow.