Nonliear wave interactions in realistic sea states
File(s)
Author(s)
Hadjigeorgiou, Demetris
Type
Thesis or dissertation
Abstract
Large surface water waves present a significant risk to offshore activities and, by extension, to human life and the natural environment. This thesis is concerned with the nonlinear wave interactions in realistic sea states that lead to the formation of extremely large wave crests. In particular, it presents numerical and laboratory investigations which were carried out to determine the physical mechanisms leading to, and the statistical distribution of, wave crests and to assess the limitations of methods commonly used to model them.
Previous research has been divided as to whether these waves are formed by the constructive interference of many freely propagating wave components, or by the modulational instability of a narrow-banded wave train. This thesis demonstrates that large wave crests are formed by the former mechanism and further enhanced by the latter. To show this, the Zakharov Equation (ZE) was used to generate long records of realistic sea states and comparisons made against published experimental data. Additionally, numerical simulations using a High-Order Spectral Method
(HOSM) presented herein show that the introduction of directional spreading of the energy reduces the probability of extreme waves, but not as rapidly as previously reported. These numerical results provide an independent validation of previously reported experimental measurements. The conclusions of this study are also extended to sea states in finite water depths.
With regards to the validity of commonly used irregular wave models, a validity diagram is presented from which the most appropriate theory can be selected, given a set of input parameters. The diagram was calculated using numerically generated idealised wave groups and tested against realistic wave groups from both numerical Monte Carlo simulations and experimental measurements. For the assessment of velocity depth profiles, a new set of experimental data was gathered. By comparing the data to the available models, the Molin (2002) model and a Lagrangian method were found to be the most accurate.
As a result of the above studies, the physical mechanisms that lead to the formation of large wave crests have been identified and the most appropriate methods for the modelling of their surface elevation and underlying particle velocities determined.
Previous research has been divided as to whether these waves are formed by the constructive interference of many freely propagating wave components, or by the modulational instability of a narrow-banded wave train. This thesis demonstrates that large wave crests are formed by the former mechanism and further enhanced by the latter. To show this, the Zakharov Equation (ZE) was used to generate long records of realistic sea states and comparisons made against published experimental data. Additionally, numerical simulations using a High-Order Spectral Method
(HOSM) presented herein show that the introduction of directional spreading of the energy reduces the probability of extreme waves, but not as rapidly as previously reported. These numerical results provide an independent validation of previously reported experimental measurements. The conclusions of this study are also extended to sea states in finite water depths.
With regards to the validity of commonly used irregular wave models, a validity diagram is presented from which the most appropriate theory can be selected, given a set of input parameters. The diagram was calculated using numerically generated idealised wave groups and tested against realistic wave groups from both numerical Monte Carlo simulations and experimental measurements. For the assessment of velocity depth profiles, a new set of experimental data was gathered. By comparing the data to the available models, the Molin (2002) model and a Lagrangian method were found to be the most accurate.
As a result of the above studies, the physical mechanisms that lead to the formation of large wave crests have been identified and the most appropriate methods for the modelling of their surface elevation and underlying particle velocities determined.
Version
Open Access
Date Issued
2018-09
Online Publication Date
2021-01-31T00:01:27Z
2021-03-11T12:00:15Z
Date Awarded
2019-02
Copyright Statement
Creative Commons Attribution NonCommercial ShareAlike Licence
Advisor
Swan, Chris
Christou, Marios
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)