Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.714708
Title: Numerical investigation of freak wave effects on offshore structures
Author: Sun, Mengshi
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2017
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Abstract:
The freak wave is extremely dangerous to offshore structures due to its unexpected high wave height and strong nonlinearity. Although increasingly more attention is paid to the investigations of freak wave, the principle of its generation mechanism and the factors that contribute to its occurrence remain unclear. Also, few efforts were exerted to investigate the interactions between offshore structures and a freak wave such as wave run-up and slamming force. In this present work, both the two dimensional (2D) and three dimensional (3D) numerical wave tanks are established based on Navier-Stokes equations for viscous, incompressible fluid by CFD commercial software FLUENT. At first, the regular waves are generated numerically. Two different wave generation methods, paddle wave making method and the source function wave making method, are introduced. The paddle wave-making method is a physical wave generation technology which is to imitate the wave makers in the laboratory. The source function wave-making method is discussed later and the empirical formulas of the source size and source intensity are introduced. The numerical wave elevations are compared with the linear analytical results. Second, the freak waves are generated numerically. According to Longuet-Higgins wave model theory, the wave free surface can be represented by the linear sum of the individual wave components with different frequencies and random phases. Improving this wave model, the wave components have their phase adjusted, so that a large amount of energy is located at the focus position at a given time. Then two more efficient and realistic freak wave models are presented, combining wave models and phase modulation wave models, respectively. Finally, the numerical results of the shift of freak wave train focusing position and focusing time are analysed, and the time history of wave elevations are compared with the analytical results. Third, a 3-D numerical wave tank is established to perform the interactions between a freak wave train and a single cylinder or a pair of two cylinders. How the focused wave parameters, including wave steepness, frequency bandwidth, focused position and the distance between the two cylinders, affect the freak wave run-up and total slamming forces on the cylinders are investigated. Finally, the hydrodynamic behaviour of a rectangular body in roll motions under both freak wave excitation and internal flow sloshing is investigated in a CFD numerical wave tank. In this study, three different freak wave conditions are considered, and two different water levels are investigated. The comparisons of numerical regular wave elevations and first order analytical results show that the current CFD numerical wave tank based on computational fluid dynamic commercial software FLUENT has a good capacity in sea water waves simulation. The focused wave parameters, such as frequency bandwidth and input wave steepness, have an obvious effect on the nonlinear behaviour of a focused wave group. This nonlinear behaviour will not only downstream shift the focused position and focused time, but also change the wave elevation at the focused position largely. The increased nonlinear behaviour of a focused wave group will increase the wave run-up along a fixed vertical cylinder at the incident wave facing direction largely. The bigger nonlinear behaviour of a focused wave group can result in larger rolling motion amplitude for a floating rectangular body, however the anti-rolling behaviour is obvious for the low filling case.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.714708  DOI: Not available
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