Numerical Simulation Of Rogue Waves And Their Action On Structures

Rogue waves are waves with gaint crests and great harzard, which have attracted a lot of concern because of the wave-related accidents. However, since rogue waves occur temporally and occasionally, the practical records are limited, and therefore their research is still unmature. Usually, rogue waves appear suddenly without any prediction, but they are short-lived and disappear in a short time. Rogue waves could be found in deep water, shallow water or finite-depth water, under storms or common sea states, with or without strong currents. Given the special characteristics and severe destruction of rogue waves, it has both theoretical and practical interest to study their physical mechanics, simulation methods and wave-structure interaction.This research is financed by the National Science Foundation of China “Dynamic physical mechanisms of rogue waves and their nonlinear interactions with deep-ocean platforms”. The purpose of this work is to explore the formation of rogue waves under exsiting research, to efficiently generate rogue waves of various categories, and to give a deep study of the rogue-wave-structure interactions, especially transient impact issues with complex surface flows.In this paper, a comprehensive discussion about the existing models of rogue waves is given, a probability-based superposition method and a breather-based nonlinear second-order solution are proposed. In the meanwhile, using the numerical wave tank(NWT), the paper generates various rogue waves and investigates their interaction with structures(for instance, a cylinder and flat plates). During research, the paper has obtained a number of achievements in various aspects, including the physical models of rogue waves, the wave-absorption technique in NWT, the wave overtopping model, and the simulation algorithm of fluid-structure interaction(FSI) problems. The main work of this paper is briefly introduced as follows:(1) An overview is made on existing rogue-wave models, the generation methods and the related research into wave-structure interaction. The characteristics and drawbacks of existing research and methods are analyzed, and the aspects to be improved are discussed.(2) A two-dimentional NWT, which solves N-S equations with the finite difference method and reconstructs free surface with the VOF-Youngs method, is established using the Visual Basic programming language. In order to preserve their entire nonlinearity, the NWT generates rogue waves by setting the velocities of the wave-coming boundary.(3) The focusing model of rogue waves is discussed from the point of view of the inversion of the initial distrubence, and the propagation of rogue waves which present Gauss-type envelopes are discussed.(4) The traditional momentum-source method and relaxation method are easy toconduct. However, owning a constant absorption parameter, their adaptability to the variation of the fluid field is weak. Sometimes, the absorption effect is not satisfactory for problems characterized by large mass transportation. Given the above-mentioned drawbacks, a new wave-absorption method, i.e., the conserved absorption method, is proposed on the basis of the traditional relaxation method by introducint a time-dependent absorption parameter. The conserved absorption method works by positioning mass sources within the absorption region, and determines the absorption parameter by solving the mass-conservation equation. The new method adapts to the fluid field of every moment, and can well handle the mass transportation phenomenon, which may occur during the propagation of rogue waves and sea currents. It also improves the efficiency of wave absorption and lowers the extension of the absorption region.(5) The traditional wave-superposition model is capable of generating freak waves efficiently, and thereby is widely utilized under laboratories and numerical wave tanks. However, using the traditional method, the wave-component number and energy proportion are usually empirically decided. If the energy proportion of the focusing wave train is large, significant deviation from the realistic sea state may be found. On the contrary, a small proportion may lead to low simulation efficiency. Given the above drawbacks, a probability-based superposition model is proposed on the basis of the conventional superposition model, considering the inherent randomness of the fluid field. By giving the occurrence probability of rogue waves under various parameter values, the probability-based superposition model reveals the influence of the energy proportion and wave-component number, points out the method of lowering the focusing-wave energy, and improves the similarity to practical sea states.(6) Being a widely-used nonlinear model of freak waves, the Peregrine breather of cubic NLS equation has been successfully generated under laboratory. In this paper, the Peregrine-breather-based rogue waves are simulated within the NWT of the paper. The formulas of life time and travelling distance of rogue waves are proposed. The second-order Stokes component solution of rogue waves under finite-depth water is proposed and simulated(7) The green water phenomenon, induced by rogue wave overtopping a rigid horizontal plate, is simulated and studied in the NWT. The traditional Ritter’s solution of the dam-breaking model is improved, considering the influence of the water depth at the deck front. Based on the improved dam-breaking model, a new model, i.e., the combined wave-dam-breaking(CWDB) model is proposed, considering the energy focusing and on-deck propagating effect of rogue waves. The validation of CWDB model is performed by conduction a series of numerical tests. Using the CWDB model, the on-deck water depth and sectional-averaged velocity are successfully predicted.(8) The solution of flow around vertical cylinder is proposed, for rogue waves with Gauss-shape envelopes. Based on the solution, the wave-induced force and moment upon the cylinder are given and comprehensively studied. A far-field approximation of the diffraction potential is proposed. The diffraction issue is numerically studied using a frequency-domain potential solver. The numerical result is compared against the analytical result for the purpose of validating the theretical formula.(9) The rogue-wave-structure interaction problems, such as green water and wave impact, are often coupled with strong impulsive transient load and complex surface flows. Inspired by the concept of the added-damping effect of the potential-flow theory, a new FSI algorithm, i.e., a SIMPLE-based monolithic implicit method(SBMIM), is proposed for the simulation of FSI issues with strong added-mass effect and complex surface flows. The rogue-wave overtopping incidents and wave-induced slamming problems are simulated using SBMIM, considering the FSI effect. The hydroelastic response and wet modes of structures are studied, together with the local pressure distribution of the fluid field.