Numerical Analysis On The Propagation And Interaction Of Internal Solitary Waves

The internal solitary waves(ISWs)that exist in the ocean have a wide range of occurrence,with large amplitude and strong nonlinearity,and are a kind of catastrophic marine environmental factor.Satellite remote sensing images and field observations have shown that the interaction of ISWs is a common phenomenon in sea areas where ISWs occur frequently.The frontal collision and overtaking of ISWs are special cases of the "weak" and "strong" interactions of ISWs,respectively.After realizing the propagation and interaction of solitary waves(SWs)by Boundary Element Method(BEM)in the theory of potential flow,this dissertation numerically simulates the propagation,frontal collision and overtaking of ISWs in a two-layer inviscid fluid with strong stratification.This dissertation firstly uses BEM to numerically simulate the frontal collision of nonlinear SWs and SW reflection at a vertical wall.The waveform evolution is obtained,the results of run up,wall residence time and instantaneous pressure force on the wall are compared with published results,and consistent conclusions are reached.After realizing the interaction of SWs,the Multi-Domain Boundary Element Method(MDBEM)is used to study the frontal collision of ISWs in two layers of strong stratified fluids,and the effect of the different normalized initial wave heights,the density ratio of the upper and lower fluids,the water depth ratio,rigid-lid(RL)and nonlinear free-surface(FS)boundary condition of the upper fluid and symmetrical and asymmetrical collision are discussed.Studies have shown that the front collision of ISWs is the asymmetric and non-linear collision process.After collision,the waveform will penetrate the interface,gradually evolve into a small dispersive trailing wave,and tilt slightly backward with respect to the direction of its propagation and the wave height will decrease slightly.During the collision of ISWs,the attachment time is shorter than the detachment time and the residence time is very long for waves of small amplitude,large depth ratios and density ratios of the lower and upper fluid layers.Considering the free surface of the top layer whether or not makes contributions to the run up and the residence time.This paper also discusses the manifestation of free surface waves during the collision of ISWs under nonlinear free surface conditions.The research shows that the waveform evolution of ISWs and free surface waves are significantly different in form and time.During the interaction of surface waves,there will be ups and downs.Finally,the MDBEM and Fourier Spectrum Method(FSM)of the KdV equation are used to study the waveform evolution of ISWs overtaking.It finds that the overtaking of ISWs can be divided into three types according to the ratio of initial amplitude and the classification criteria are consistent with classification in the overtaking of SWs.There are many differences in the amplitude variation curves obtained by the two methods,but the minimum amplitudes of the action center are both located near the boundary point of whether the two ISWs interact.The waveform evolution of the free surface waves and ISWs is almost the same in shape and time.The waveform evolution,wave amplitude change,nonlinear characteristics and surface wave characteristics generated by the interaction of ISWs during the frontal collision and overtaking obtained in this paper can provide important research basis for detection,and parameter inversion of ISWs and safe operation of underwater vehicles.