The Effects on Water Particle Velocity of Wave Peaks Induced by Nonlinearity under Different Time Scales

The water particle velocity of the wave peaks is closely related to the wave load borne by offshore structures. It is of great value for marine disaster prevention to study the water particle velocity of nonlinear extreme waves represented by Freak waves. This study applies the High-order Spectral Method (HOS) numerical model to analyze the characteristics and influencing factors of the water particle velocity of Freak wave peak with two different generation mechanisms under the initial condition of a weakly modulated Stokes wave train. Our results show that the water particle velocity of the wave peak increases linearly with wave height and initial wave steepness in the evolution stage of modulation instability. While in the later stage, the relationship becomes exponential. Under the condition of similar wave heights, the deformation degrees of Freak waves with different generation mechanisms are distinct, the deformation degree of modulation instability stage is smaller than that of the later stage. The water particle velocity of the wave peaks increases with the deformation degrees. Furthermore, the correlation between wave peak height and water particle velocity is a quadratic function. This provides a theoretical basis for further understanding of nonlinear waves and the prediction of marine disasters.

Numerical Analysis of Wave Characteristic In the Freak Wave-- “New Year Wave” Formation

Freak waves are both extremely large waves and highly transient time. Such a wave may lead to damage of ships to deaths. In this paper, to describe the connection between freak wave and wave essential factor, we use WAVEWATCH III model simulating “New Year Wave” in the North Sea to explore freak wave, with the importing of ECMWF re-analysis wind field. By this way, we successfully simulate the formation of freak wave in the random wave. Analysis shows large wave steepness and small directional spread angle are necessary conditions for freak waves to easily occur. By analyzing the wave spectrum, it is found that the wave energy is distributed in a small range, and the propagation direction is relatively concentrated.

EXCITATION OF FREAK WAVE BY NATURAL OSCILLATIONS OF THE WATER BODY

In linear theory the formation of extreme waves their existence is interpreted as a local superposition of surface monochromatic waves. Natural water areas are resonators that have their own set of natural oscillations – standing waves of stable spatial structure and fixed period. In the spectra of waves of many water bodies of World Ocean observed double high waves, this is explained by the tidal-seiche resonance. During a storm, the energy of natural oscillations increases ten times the background energy, during a tsunami it can increase up to three orders of magnitude. Examples of the effects of natural oscillations on the coast are given, and it is reported about the increased probability of the occurrence on the coast freak waves. Additionally, it is noted that natural oscillations in water mass are a normal state for any body of water at any time of its existence. The corresponding indices of the water fluctuations of the water basins are given. The events of extreme waves during the accidents at DniproHES (Zaporizhia) on August 18, 1941, and the Kurenivsky dam (Kyiv) on March 13, 1961, are presented. The excitement of the freak wave can be interpreted as enhancing the natural oscillations of the water basin, represented by standing waves of stable spatial structure, fixed period and high probability of waves in the water body. This does not contradict the linear theory of the resonant formation of abnormally high waves. The purpose of the article is to investigate possible sources of the excitement of freak waves, the results are proposed to be implemented in the development of countermeasures to the destructive process. However, the waves carry out both destructive and creative work. A task is presented, which involves the development of measures to stimulate extreme waves. This will increase electricity generation. Affiliation of dam-break waves to freak waves can be doubtful. However, they formally correspond to the classical condition of double exceeding the significant wave height. Most water basins are integral anthropogenic sites. The variability of both natural and anthropogenic environments forces the overriding of systematization and definition. It is proposed to attribute extreme waves of dam-break waves to freak waves.

Numerical Simulation of Inline Forces on a Bottom-Mounted Circular Cylinder Under the Action of a Specific Freak Wave

Freak waves pose a great threat to the tension-leg platforms (TLPs) and monopile foundations of offshore wind turbines (OWTs), which necessitates comprehensive investigations on the characteristics of freak waves and the wave actions on those offshore renewable energy structures with circular cylinder. The recorded freak wave series “New Year Wave” (NYW) was numerically simulated using the Computational Fluid Dynamics methods. The compensation measure was adopted to effectively improve simulation accuracy. Under the action of the NYW, the inline forces and secondary load cycle (SLC) on a vertical-mounted cylinder, as the classic form for the TLPs and foundation of OWTs, were fully addressed. The simulation results were compared with the empirical formulations and experimental data to reveal the differences and the possible causes. The development of SLC was found to be closely related to the downstream vortex and return flow, which induces the reduction of the wall pressure and thus the inline force. The maximum inline forces vary with the cylinder position relative to the wave peak, and the simulation results reveal that the linear inline forces calculated by Morison formulation may be less than 65% of the total wave forces.