Internal Solitary Wave Reflection And Interaction Experiments And The Propagation Characteristics Of Internal Solitary Waves In The Northern South China Sea

Internal solitary waves are ubiquitous phenomena in the oceans and they can pose serious effects on the ocean environment and ocean engineering. Korteweg-de Vries (KdV) equation has been a widely accepted model for the research on internal solitary waves since 1960. The theory of solitons’reflection and interaction were first developed based on the solitons with shape of sech2, which is a solution of KdV equation. Under specific conditions, the reflection of solitary waves on a rigid wall can produce a fourfold run-up (Mach stem) at the intersection area and the crest of the new wave can grow with time. However, there are rare experimental studies of this phenomenon of internal solitary waves in a two-layer fluid system. On the other hand, KdV equation with assumption of two-layer stratification is the most widely used to simulate solitary waves. The amplitude of internal solitary waves can be derived from the observed maximum horizontal current induced by internal soltitary waves. A potential early warning method can be designed based on KdV equation and a combination of in-situ current measurements and stratification data.This dissertation consists of two major parts:In the first part, the reflections and interactions of internal solitary waves are simulated in a two-layer fluid in a laboratory tank. The experimental results are compared with the classic theories. According to the experiments, the method of gravity collapse is more convenient to generate an internal solitary wave than gravitational current from the mixed fluid area. With the same stratification and amplitude, the reflection type of an incident internal solitary wave is determined by the incidence angle, which is similar to the theory of surface solitons. However, the maximum amplitude of the new-generated stem is much smaller than theoretically predicted. The step angle is larger than theoretical value which demonstrates the growth rate of the stem is faster. The four-fold amplification is not observed and the reflection angle seems to be a varying parameter instead of a constant.In the second part, in-situ current measurements by three moorings are used to conduct analysis for propagation speeds of internal solitary waves in the Liwan Block. Five typical internal solitary waves are derived from the in-situ current data and four of them are captured by two synchronous moorings. The water stratification is based on W0A13 data. One of the internal solitary waves is test with the background currents to show the effects on its propagation speed and it is found that the deviation is about 5%, which is accurate enough for early warning. The depth of maximum first-mode function is chosen as the upper layer thickness and an early warning method of large internal solitary waves in the Liwan Block can be achieved by inspecting upper layer water current and using KdV model. In addition, the loads on the’HYSY981’platform induced by typical an internal solitary wave are demonstrated based on Morison’s equation and other empirical formulas, which may provide some reference for safety control on the rig.