Research On The Formation Mechanism And The Surface Signature Characteristics Of The Internal Wave Generated By A Submerged Body

The fluid density commonly stratifies in the ocean due to the vertical distribution of temperature and salinity.Coupled with a certain source of excitation,the surface wave and internal wave will be generated simultaneously.Compared with the surface wave,the internal wave has the characteristics of a long period and large amplitude.The emission of internal waves by submerged bodies in a stratified fluid is an important phenomenon in the ocean.It is possible to detect the submerged body through the internal waves.This interesting phenomenon has a wide application for the environment and navy and is a fundamental problem of stratified flow physics.On the basis of summarizing recent research situation for such subjects,the thesis presented a combined experimental,theoretical and time-frequency analysis method to study the formation mechanism and characterization of surface waves generated by submerged bodies in a stratified fluid.The thesis is organized as follow.To study formation mechanism and characterization of the internal wave excited by a submerged body,an innovative time-frequency analysis method for internal waves is introduced.A series of experiments are performed for the internal waves generated by the ball and SUBOFF model.A ball and a SUBOFF model were used in the experiments and were driven by the towing line or by its self-propelling system.The internal wave elevation signals were captured by the conductivity probes.Based on the time-frequency analysis method,results show that the experimental spectrograms of the wave elevation signals contain four portions:the leading,reflected,nonlinear and low frequency portions.The portion in the leading and reflected regions indicate the Lee waves and their reflections,which relates to the volume effect.The nonlinear portion emerges when the Froude number near Fr_p,which relates to the weak nonlinear effect.The low frequency portion is the wake waves generated by the large vortex structure behind the submerged body and the intensity of the low frequency portion is tightly related with the net momentum.To study the surface signature of the internal wave excited by a ball,a series of experiments are conducted in a tank sufficiently wide.In the experiment,the particle image velocimetry technology and the wave elevation acquisition system are used to measure the velocity field and wave height signal of the surface wave.Results show that the surface wave contains three components,including the internal wave mode,surface wave mode and the wakes.The three components were roughly separated by the filter function.As the Froude number increases,the amplitude of internal mode wave decreases gradually,while the amplitude of the surface wave mode and wakes increases linearly.The amplitude of the internal wave mode and wakes decreases when the pycnocline is getting further away from the free surface.The amplitude of the surface mode wave and the wakes deceases significantly when the ball is located at a deeper position.To further study the surface signature,a modified theoretical model of a source moving in a stratified fluid is proposed,and the rigid lid surface assumption is replaced by the free surface condition in the modified model.This model can predict the internal and surface waves excited by a volume source with arbitrary density profiles.The model adopts a linearized governing equation and free surface condition and considers a moving volume source in the equation.By solving the eigenvalue problem with a free surface,the equation is solved by combining the Green’s function method.The vertical displacement and velocity field of moving objects in a stratified fluid is obtained by using Fourier transform.Based on the model,the wave characteristics that are analyzed include the half-wedge angle,crestlines,the maximum amplitude,and the apparent angle.The surface waves are related to the factors of the stratifie fluid and the effects of these factors have been discussed.To study the surface signature in full scale,a series of simulations are carried out based on the modified theoretical model.The vertical distribution of ocean salinity,temperature,density and Brunt–Vaisala(BV)in the Dongsha sea area,South China is introduced in detail.Based on the actual sea conditions in Dongsha sea area,the surface wave excited by a slender volume source with 100m long and 10m diameter sailing at different depths and speeds is simulated by using the theoretical model.The characteristics of the surface wave elevation field,velocity field and surface divergence field are studied.The results show that the amplitude of internal wave mode deceases if the BV frequency becomes smaller,the pycnocline is getting further away from the surface,or the submerged body is getting further away from the pycnocline.The pycnocline of the South China Sea is thicker and closer to the surface in summer,resulting in the bigger amplitude of internal wave mode while winter is the contrary.