Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Civil and Environmental Engineering | en_US |
| dc.contributor.advisor | Dong, You (CEE) | en_US |
| dc.creator | Wu, Qian | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/13375 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Nonlinear evolution of ocean extreme waves over varying bathymetries | en_US |
| dcterms.abstract | The incidence of anomalously large waves in coastal regions poses serious coastal hazards, particularly to coastal structures and human activities. Of scientific research interest is the formation mechanism of these extreme ocean waves. One of the mechanisms is believed to be the nonlinear wave evolution over abrupt depth transitions (ADTs) in the bathymetry. To elucidate the role of ADTs in triggering extreme waves, it is important to investigate the strong nonlinear wave-wave and wave-bathymetry interactions for large waves. However, few studies have been found in the literature to investigate the nonlinearity of extreme waves over varying bathymetries. Existing linear or weakly nonlinear models may not be able to reveal strongly nonlinear effects. This motivates the development of more advanced models to deal with such problems. This thesis aims to develop a fully nonlinear numerical model to describe the highly nonlinear evolution of waves over varying bathymetries and to investigate the characteristics of higher harmonic elevations induced by bathymetry. A temporal and spatial analysis framework for subharmonics and superharmonics of wave elevations is employed. Focused wave groups, typical representative extreme wave conditions, are adopted in this thesis as the incident wave. | en_US |
| dcterms.abstract | In the past decade, fully nonlinear numerical models have been developed to confirm that abrupt depth changes can modify the statistical distribution of surface elevations. However, few of them studied the characteristics of each harmonic on the water depth transitions. In addition, most topographies are limited to simplified shapes, such as slopes and infinite steps. To address these issues, an efficient solution framework to the two-dimensional Euler equations by a conformal mapping method is developed, which eliminates the singularity at the corners of the sudden changes. This numerical model is capable of demonstrating nonlinear wave propagation in both temporal and spatial domains. Specifically, it facilitates the evolution of extreme waves and their interaction with the bathymetry. To validate the model, experiments were conducted at the laboratories of the Hong Kong Polytechnic University and the Southern University of Science and Technology. Comparisons of the surface elevation, energy spectrum and wave scattering (wave reflection and transmission) show high confidence in its capability and accuracy. | en_US |
| dcterms.abstract | Significant second-order effects have been identified in the mechanism of occurrence of extreme waves. However, the study of second-order subharmonics and superharmonics (bound and free waves) at ADT is few. Thus, through the experimental and numerical study in this thesis, a detailed evolution of the second-order harmonic elevation is demonstrated. The second-order subharmonics and superharmonics are separated to assess their contributions to the statistical distribution. Additionally, the effects of parameters such as wave steepness and relative wave- length of both the monochromatic and extreme waves are further illustrated. Generation of higher harmonics in the shallower region on the ADT step is demonstrated, where a high asymmetry of surface elevations on the upstream junction is observed. Subharmonics occur due to the increase of mean water level, on the contrary, they weaken the asymmetry of wave profiles. The total increase of kurtosis interprets a physical formation mechanism for a higher probability of extreme waves at ADTs. Results also reveal energy transfer among superharmonics as well as between the subharmonics and superharmonics on the water depth transitions. | en_US |
| dcterms.abstract | With particular bathymetries and wave conditions, there might occur extremely low or high reflection, corresponding respectively to wave trapping and Bragg resonance. These features have been adopted in the design of breakwaters and were studied widely with regular and random waves. However, the resonance of extreme waves and their nonlinear evolution across the entire spatial domain remains largely unexplored. Thus, the wave trapping with monochromatic waves and the Bragg resonance with focused wave groups are investigated in this study. The fully non-linear numerical model is first validated by linear theories and experimental data in the study of wave resonance. The nonlinearity at the trapped frequencies obtained from the linear model is assessed. Unlike the importance of kurtosis at ADTs, the skewness can be a significant parameter in discussing trapped wave progress. Then, the focused wave propagation over three types of periodic bottoms is simulated, namely ripples, bars, and steps. The optimal conditions of high Bragg reflection are proposed. Nonlinear analysis of the reflection coefficient confirms the existence of a secondary Bragg resonance. | en_US |
| dcterms.abstract | Overall, this research provides methodologies to assess the hydrodynamic characteristics of extreme waves over varying bathymetries. Compared with the traditional numerical models, the developed fully nonlinear numerical model presents the complete interplay between waves and transitions with a very high efficiency. Laboratory experiments validate the simulation results and add measured data to the literature. From the perspective of subharmonics and superharmonics, the analyzed results provide more insights into the mechanism of extreme wave generation. Additionally, the findings highlight the occurrence of Bragg reflection over different topographies and could make contributions to the design of coastal breakwaters and improvement of the potential detection in the future. | en_US |
| dcterms.extent | xxix, 201 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2024 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Rogue waves -- Mathematical models | en_US |
| dcterms.LCSH | Ocean waves -- Mathematical models | en_US |
| dcterms.LCSH | Wave mechanics | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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