Study Of Ocean Internal Wave Propagation Mechanism Based On Spatial Chirp

Ocean internal wave is a kind of wave which occurs in the density stratificationsea water, the most common one of which is tide-generated internal wave.tide-generated internal wave appears in the world’s oceans and marginal seas, whichis medium and small scale phenomenon of ocean internal waves. The development ofthe research of oceanic internal waves would greatly promote the developments ofocean acoustics, ocean engineering, military oceanography etc. The south China sea isa prone area of ocean internal waves due to the complexity of the terrain environment,and has been paid close attention to by the scholars. Researches on internal waves aremainly focused on the generation, transmission and extinction of internal waves.Especially different conclusions are drawn about the generation mechanism andnonlinear effect and dispersion effect on the propagation of internal waves. In thispaper, the generation and spread of the ocean internal wave are studied numericallyby introducing spatial chirp. The thesis are arranged as follows：Firstly, fully nonlinear Schr dinger (NLS) equation describing large amplitudespread is established. On the foundation of incompressible, inviscid and irrotationaltwo-layer model, the work starts with the Laplace formula and Bernoulli formulafrom hydromechanics. By introducing the multiple scales and with the method ofperturbation expansions, the governing equation could be transformed into the fullyNLS equation. Meanwhile, the analytic solution and analyze the physical significanceof some parameters in the equation are obtained.Secondly, the effects of dispersion and nonlinearity in deep sea are tried toexplain with the idea of spatial chirp. Assuming the initial wave is Gauss type andhyperbolic secant type respectively, the functions of dispersion-caused chirp and nonlinearity-caused chirp are resolved from the fully NLS equation. Then evolutionsof dispersion-caused chirp, nonlinearity-caused chirp and total chirp are analyzednumerically and the conclusions are made. Finally, the simulation is applied in exactocean fields in the south China sea, together with MODIS images. During the spreadof internal waves from deep water to shallow water, the dispersion effects increaseand then decrease, which leads to broadening internal wave form and decreaseamplitude; At the same time nonlinearity effects increase during the propagation,which generate new spectrum component and not change the amplitude, lead tonarrow waveform.Finally, the mechanism of internal solution waves generated in Luzon strait isdiscussed by the gain formed in the modulation instability. The largest gain formed inthe internal tidal wave modulation instability condition is derived based on fully NLSequation, and considering the water depth, stratification and the initial amplitude.Analysis of modulation instability in the Luzon strait area, and the tidal wave in theLuzon strait evolved into a process of internal solitary wave is simulated.