Study On The Generating Mechanism,Shape And Propagating Mode Of Crescent Waves

The crescent wave is composed of horseshoe-shaped arc wave surface.It is a very strong three-dimensional non-linear wave.When the wave is generated,it has broken or critically broken.It can produce strong destructive effect on offshore buildings.Therefore,it is of great significance to study the mechanism of crescent wave formation for the design of marine structures.In this paper,the theoretical analysis of crescent wave generation is given.The generation,evolution and waveform characteristics of different types of crescent waves including L2,propagating L1,oscillating L1 and other types are studied through physical model experiments,as well as the influence of different types of unstable interaction on the generation and evolution of crescent wave.The long-term evolution is simulated by using accurate dispersion numerical model.The waveform characteristics of crescent wave and the influence of wave basin length on the evolution of crescent wave are discussed.Firstly,the mechanism of crescent wave is introduced according to the linear instability theory of Stokes wave,and different kinds of crescent wave are classified.The types and waveforms of crescent wave are predicted theoretically.Through the linear instability theory of Stokes wave,the experimental techniques for generating different types of crescent wave are designed,and the measurement of arc,transverse wavelength and propagation angle of crescent wave in the experiment is given.The accurate dispersion numerical model is introduced and compared with the experimental results of crescent wave.An experimental study of L2 type crescent wave which is the most easily generated was carried out in indoor wave basin.The generation conditions of L2 type crescent wave,critical wave steepness,spatial evolution characteristics of amplitude and phase,waveform reproduction,geometric characteristics of waveform,variation characteristics of waveform along basin and influence of water depth on waveform were given.The effects of class I and class II instabilities on the evolution of L2 type crescent waves are discussed.A completely nonlinear exact dispersion model is used to simulate the L2 type crescent wave,and the characteristics of long term evolution of crescent wave are studied.The influence of basin length on the evolution of L2 type crescent wave is also discussed.A new type of crescent wave which is named propagating L1 type crescent wave is found.The generating conditions and classification of L1 type crescent wave are given.The difference between L2 type crescent wave and this kind of crescent wave is compared.The spatial evolution of amplitude and phase of propagating L1 type crescent wave is studied by identifiable and non-identifiable modes.The crescent wave waveforms of different development stages are shown by waveform reproduction.The influence of the amplitude and phase of different wave components on the waveform is studied;the geometrical characteristics of the waveform are discussed;the comparison results of the propagation angle between experimental measurement and theoretical calculation are given;the numerical simulation of L1 type crescent wave is carried out by using the accurate dispersion model,and the effects of long term evolution on the evolution and development of crescent wave are studied.Three methods of generating oscillating crescent waves in laboratory are given,and the differences of the spatial evolution characteristics of the spectral amplitude and phase of the oscillating crescent waves produced by three methods are compared.The oscillating crescent waves in the experiment are reproduced and compared with L2 type crescent waves which has the same symmetrical form.The geometric characteristics of oscillating crescent waves are discussed.The oscillating cycle of oscillating crescent waves is studied.The influence of water depth on the oscillating crescent wave is discussed.The exact dispersion model is used to simulate the oscillating crescent wave for long term evolution.The effects of basin length on the evolution and development of the oscillating crescent wave are studied.A series of experiments are carried out to study the characteristics and evolution of crescent waves generated by two types of unstable modes.The spatial evolution of amplitude and phase of crescent wave generated by two kinds of unstable modes is given,the differences between the two modes are discussed,and the effects of different initial disturbance frequencies on the waveforms are analyzed.The interaction between class I instability and class II instability in the evolution of crescent waves is discussed,and their effects on the evolution of crescent waves are analyzed.The interaction of various instability modes in the evolution of crescent waves is analyzed and compared with the theoretical prediction results.