The Study On Modeling Technology Of Arctic Sea-ice Acoustic Waveguide

With the continued global warming,polar science has become one of the most visible research fields of the world.Progresses have been made on Polar acoustics with the efforts of scientists from worldwide.With the stereoscopic monitoring capacity in polar regions was established as a national strategic need of China,the acoustic theory and technologies applicable to Arctic sea-ice,which has long been overlooked as a branch of acoustics,is now dawning more and more attention.The sound propagation in Arctic sea-ice is dominated by the waveguide phenomenon,the research of which faces grave challenges due to the complex features of sea-ice,the bi-phase yet asymmetrical boundary conditions,and the inaccessibility for in-situ experiments caused by the harsh environment.In particular,the acoustic impedance of metal is much greater than water in ultrasonic technologies,while it is extremely close between sea-ice and sea-water and the density of sea-water is greater than that of sea-ice,thus it is difficult to neglect the serious energy leakage.Aiming at an effective and precise modeling of the acoustic waveguide in sea-ice,this paper started from four parts:(1)establish the theoretical model of the ice acoustic waveguide,(2)solve the ice acoustic waveguide model,(3)verify the model based on numerical simulation method,(4)design and implement the experiment on the Songhua River ice acoustic waveguide.Based on these,theoretical research and verification analysis were carried out on the sound propagation in ice and the acoustic leakage caused by fluid-solid coupling conditions.First,a free elastic plate model and a sea ice waveguide model with symmetric and asymmetric ice-water coupling boundaries were established.Also,Methods for solving the model in real domain such as traditional bisection method and non-fixed resolution self-search bisection method were introduced.After that,to solve the strong sound energy leakage caused by the proximity of acoustic impedance between ice and water,complex solving methods such as local peak search method and spectral method were developed.This paper also proposed a single-mode peak self-search method in complex space,which can efficiently solve the model for a single mode.Subsequently,based on the spectral element numerical simulation method(SEM),the comparison with theoretical results were discussed.The Arctic sea ice waveguide model was established by SEM,which is more suitable for the numerical simulation of fluid-solid coupling.Comparing with the theoretical modeling and solution,we concluded that it is necessary to consider the leaky mode.That is,the sound propagation problem in sea ice waveguide should focus on the leaky mode,and the influence of water depth on it can be ignored to a certain extent.Finally,the Songhua River ice-acoustic waveguide experiment was carried out to further verify the theoretical modeling and numerical simulation.On this basis,the author found that the concepts of flexural wave mode and QS mode of ice-acoustic waveguide were confused in some previous literatures.At low frequency(<800 Hz),the energy of flexural wave is seriously attenuated and difficult to observe.Therefore,the high-energy modal signal received in the field test is the QS mode,not the flexural wave mode.The research on ice acoustic propagation and ice acoustic waveguide is of great positive significance for the understanding and development of polar regions.The combination of ice acoustic propagation and ice acoustic excitation technology research can improve the quality of acoustic communication across the ice medium,and then provide help for the detection and location of targets under ice and serve the final construction of the integrated network of polar exploration.Therefore,how to establish a more refined and realistic model of polar sea ice is one of the focuses of future polar ice acoustic research.