Publications
93 results found
Karmpadakis I, Swan C, Christou M, 2019, Laboratory investigation of crest height statistics in intermediate water depths, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 475, Pages: 1-24, ISSN: 1364-5021
This paper concerns the statistical distribution of the crest heights associated with surface waves in intermediate water depths. The results of a new laboratory study are presented in which data generated in different experimental facilities are used to establish departures from commonly applied statistical distributions. Specifically, the effects of varying sea-state steepness, effective water depth and directional spread are investigated. Following an extensive validation of the experimental data, including direct comparisons to available field data, it is shown that the nonlinear amplification of crest heights above second-order theory observed in steep deep water sea states is equally appropriate to intermediate water depths. These nonlinear amplifications increase with the sea-state steepness and reduce with the directional spread. While the latter effect is undoubtedly important, the present data confirm that significant amplifications above second order (5–10%) are observed for realistic directional spreads. This is consistent with available field data. With further increases in the sea-state steepness, the dissipative effects of wave breaking act to reduce these nonlinear amplifications. While the competing mechanisms of nonlinear amplification and wave breaking are relevant to a full range of water depths, the relative importance of wave breaking increases as the effective water depth reduces.
Latheef M, Swan C, Spinneken J, 2017, A laboratory study of nonlinear changes in the directionality of extreme seas, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 473, ISSN: 1364-5021
This paper concerns the description of surface water waves, specifically nonlinear changes in the directionality. Supporting calculations are provided to establish the best method of directional wave generation, the preferred method of directional analysis and the inputs on which such a method should be based. These calculations show that a random directional method, in which the phasing, amplitude and direction of propagation of individual wave components are chosen randomly, has benefits in achieving the required ergodicity. In terms of analysis procedures, the extended maximum entropy principle, with inputs based upon vector quantities, produces the best description of directionality. With laboratory data describing the water surface elevation and the two horizontal velocity components at a single point, several steep sea states are considered. The results confirm that, as the steepness of a sea state increases, the overall directionality of the sea state reduces. More importantly, it is also shown that the largest waves become less spread or more unidirectional than the sea state as a whole. This provides an important link to earlier descriptions of deterministic wave groups produced by frequency focusing, helps to explain recent field observations and has important practical implications for the design of marine structures and vessels.
Rodriguez M, Spinneken J, Swan C, 2016, Nonlinear loading of a two-dimensional heaving box, Journal of Fluids and Structures, Vol: 60, Pages: 80-96, ISSN: 1095-8622
A numerical investigation is presented addressing the nonlinear heave response of a rectangular box. The work specifically concerns the importance of the relative body dimensions, expressed through the product of the half-beam b and the wavenumber k . When subjected to moderately steep incident waves, the second-harmonic content of the heave motion is found to be as large as 25% of the first-harmonic content. In considering the extent of this second-harmonic motion, three regimes may be defined: (i) the long wave regime, where kb≤0.4, (ii) the intermediate regime, where 0.4<kb<1.0 and (iii) the diffraction or short wave regime, where kb≥1.0. Expressed in terms of the wavelength λ=2π/k, these regimes correspond to (i) b≤0.06λ, (ii) 0.06λ<b<0.16λ and (iii) b≥0.16λ. The second-harmonic motion content is found to be particularly pronounced in regimes (i) and (iii). Perhaps surprisingly, this second-harmonic content is also found to be practically non-existent for some intermediate cases lying within regime (ii). Three sources of nonlinearity are shown to be particularly important. First, the interaction between the first-order incident waves and the first-order scattered waves is key to the nonlinear loading in regime (iii). Second, the generation of freely propagating second-harmonic radiated waves due to the body motion is important in (i). Third, the local standing wave field associated with the radiation problem is found to contribute to the loading in regime (iii). In addition, the location of the body resonance also plays a critical role in defining the extent of the second-harmonic motion content. The focus of the present work lies on a clear physical interpretation of the sources of these nonlinear loads, coupled with an analysis of the body dynamics.
Peric M, Swan C, 2015, An experimental study of the wave excitation in the gap between two closely spaced bodies, with implications for LNG offloading, APPLIED OCEAN RESEARCH, Vol: 51, Pages: 320-330, ISSN: 0141-1187
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- Citations: 40
Swan C, Sheikh R, 2015, The interaction between steep waves and a surface-piercing column, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 373, ISSN: 1364-503X
Experimental observations are presented of a single surface-piercing column subject to a wide range of surface gravity waves. With the column diameter, D, chosen such that the flow lies within the drag-inertia regime, two types of high-frequency wave scattering are identified. The first is driven by the run-up and wash-down on the surface of the column in the vicinity of the upstream and downstream stagnation points. The second concerns the circulation of fluid around the column, leading to the scattering of a pair of non-concentric wavefronts. The phasing of the wave cycle at which this second mode evolves is dependent upon the time taken for fluid to move around the column. This introduces an additional time-scale, explaining why existing diffraction solutions, based upon a harmonic analysis of the incident waves, cannot describe this scattered component. The interaction between the scattered waves and the next (steep) incident wave can produce a large amplification of the scattered waves, particularly the second type. Evidence is provided to show that these interactions can produce highly localized free-surface effects, including vertical jetting, with important implications for the setting of deck elevations, the occurrence of wave slamming and the development of large run-up velocities.
Hennig J, Scharnke J, Swan C, et al., 2015, EFFECT OF SHORT-CRESTEDNESS ON EXTREME WAVE IMPACT - A SUMMARY OF FINDINGS FROM THE JOINT INDUSTRY PROJECT "SHORTCREST", 34th ASME International Conference on Ocean, Offshore and Arctic Engineering (OMAE2015), Publisher: AMER SOC MECHANICAL ENGINEERS
Spinneken J, Christou M, Swan C, 2014, Force-controlled absorption in a fully-nonlinear numerical wave tank, Journal of Computational Physics, Vol: 272, Pages: 127-148, ISSN: 0021-9991
An active control methodology for the absorption of water waves in a numerical wave tank is introduced. This methodology is based upon a force-feedback technique which has previously been shown to be very effective in physical wave tanks. Unlike other methods, an a-priori knowledge of the wave conditions in the tank is not required; the absorption controller being designed to automatically respond to a wide range of wave conditions. In comparison to numerical sponge layers, effective wave absorption is achieved on the boundary, thereby minimising the spatial extent of the numerical wave tank. In contrast to the imposition of radiation conditions, the scheme is inherently capable of absorbing irregular waves. Most importantly, simultaneous generation and absorption can be achieved. This is an important advance when considering inclusion of reflective bodies within the numerical wave tank.In designing the absorption controller, an infinite impulse response filter is adopted, thereby eliminating the problem of non-causality in the controller optimisation. Two alternative controllers are considered, both implemented in a fully-nonlinear wave tank based on a multiple-flux boundary element scheme. To simplify the problem under consideration, the present analysis is limited to water waves propagating in a two-dimensional domain.The paper presents an extensive numerical validation which demonstrates the success of the method for a wide range of wave conditions including regular, focused and random waves. The numerical investigation also highlights some of the limitations of the method, particularly in simultaneously generating and absorbing large amplitude or highly-nonlinear waves. The findings of the present numerical study are directly applicable to related fields where optimum absorption is sought; these include physical wavemaking, wave power absorption and a wide range of numerical wave tank schemes.
Haley JF, Swan C, Gibson R, 2014, AN EXPERIMENTAL INVESTIGATION OF WAVE IMPACT LOADS ON A SLENDER HORIZONTAL CYLINDER, 33rd ASME International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS
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- Citations: 3
Latheef M, Swan C, 2013, A laboratory study of wave crest statistics and the role of directional spreading, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 469, ISSN: 1364-5021
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- Citations: 35
Katsardi V, de Lutio L, Swan C, 2013, An experimental study of large waves in intermediate and shallow water depths. Part I: Wave height and crest height statistics, COASTAL ENGINEERING, Vol: 73, Pages: 43-57, ISSN: 0378-3839
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- Citations: 29
Spinneken J, Swan C, 2012, The operation of a 3D wave basin in force control, Ocean Engineering, Vol: 55, Pages: 88-100
The present study concerns the generation and absorption of directional waves in a facility where the generation and absorption mechanism is based on force-feedback control. Many laboratories worldwide are now equipped with such technology, and the world’s largest wave basin (the David Taylor Basin at the Naval Surface Warfare Centre, Carderock, USA) is currently being refurbished utilising force-driven wavemakers. Traditionally, the wave generation in such facilities was based on an empirical transfer function. In contrast, an entirely analytic approach is presented herein. A theoretical transfer function, enabling fully deterministic wave generation in force-controlled waves basins, is derived for the first time. The theory is applicable to both flap- and piston-type wave machines. Even though the present study focuses on flap-type geometries, the results are readily adoptedto the piston case. To demonstrate the successful application of the novel transfer function, a substantial experimental study is presented. As part of the experimental work, a direct comparison to an empirical transfer function is made and the benefits of either approach are discussed. Overall, very good agreement betweenthe expected wave field and the experimental data is shown. However, some departures remain, particularly for highly directional waves and broad-banded spectra.
Bateman WJD, Katsardi V, Swan C, 2012, Extreme ocean waves. Part I. The practical application of fully nonlinear wave modelling, APPLIED OCEAN RESEARCH, Vol: 34, Pages: 209-224, ISSN: 0141-1187
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- Citations: 16
Spinneken J, Swan C, 2011, Theoretical Transfer Function for Force-controlled Wave Machines, INTERNATIONAL JOURNAL OF OFFSHORE AND POLAR ENGINEERING, Vol: 21, Pages: 169-177, ISSN: 1053-5381
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- Citations: 6
Katsardi V, Swan C, 2011, The evolution of large non-breaking waves in intermediate and shallow water. I. Numerical calculations of uni-directional seas, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 467, Pages: 778-805, ISSN: 1364-5021
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- Citations: 17
Latheef M, Swan C, 2011, WAVE STATISTICS IN NONLINEAR SEA STATES, 30th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 761-769
Katsardi V, Swan C, 2011, AN EXPERIMENTAL STUDY OF SHALLOW WATER WAVE STATISTICS ON MILD BED SLOPES, 30th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 711-719
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- Citations: 3
Roos J, Swan C, Haver S, 2010, WAVE IMPACTS ON THE COLUMN OF A GRAVITY BASED STRUCTURE, 29th ASME International Conference on Ocean, Offshore and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 365-373
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- Citations: 4
Spinneken J, Swan C, 2010, Theoretical Transfer Function of Force-controlled Wave Machines, 20th International Offshore and Polar Engineering Conference, Pages: 409-417
Spinneken J, Swan C, 2009, Wave generation and absorption using force-controlled wave machines, Pages: 397-405, ISSN: 1098-6189
Recent research on wave generation utilizing absorbing force-controlled machines has shown three key factors: (i) a theoretical transfer function between demand signal and surface elevation can be derived; (ii) high absorption efficiency can be achieved over a wide range of frequencies and (iii) little second-order spurious content is introduced when driven with a first-order demand signal. Thus far, both theoretical and experimental work has been limited to the generation of regular wave trains and hinged wave board geometries. The present study extends this work to the generation of irregular waves considering both flap-and piston-type wave machines. Whilst limiting the discussion to the effects arising at first-order, specifically addressing the theoretical transfer function and absorption efficiency, an enhanced understanding of the machine's controller is sought. The implemented absorption strategy is based on controlling the complex relationship between applied force and wave board velocity. The properties of the optimum a-causal controller for this particular strategy are investigated and a causal, hence practical, approximation is derived. A causal controller based on infinite impulse response filters (IIR) and direct optimization in the frequency domain is considered. The relative merits of this approach are compared to methods based on an approximation in the time domain. Experimental evidence is also provided to substantiate the IIR modelling approach. The results are directly relevant to the operation of many installed force-controlled wave machines, provide guidance as to the effective operation of others, contribute to the wave power debate and could be incorporated within advanced numerical wave tanks to provide simultaneous generation and absorption. Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE).
Hague CH, Swan C, 2009, A multiple flux boundary element method applied to the description of surface water waves, JOURNAL OF COMPUTATIONAL PHYSICS, Vol: 228, Pages: 5111-5128, ISSN: 0021-9991
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- Citations: 20
Christou M, Hague CH, Swan C, 2009, The reflection of nonlinear irregular surface water waves, ENGINEERING ANALYSIS WITH BOUNDARY ELEMENTS, Vol: 33, Pages: 644-653, ISSN: 0955-7997
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- Citations: 8
Spentza E, Swan C, 2009, WAVE-VESSEL INTERACTIONS IN BEAM SEAS, 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE), Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 393-403
Roos J, Swan C, Haver S, et al., 2009, AN EXPERIMENTAL INVESTIGATION OF WAVE IMPACTS ON THE DECK OF A GRAVITY BASED STRUCTURE, 28th International Conference on Ocean, Offshore and Arctic Engineering (OMAE), Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 405-414
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- Citations: 2
Spinneken J, Swan C, 2009, Second-order wave maker theory using force-feedback control. Part II. an experimental verification of regular wave generation, Ocean Engineering, Vol: 36, Pages: 549-555
This paper provides an experimental verification of the new wave maker theory outlined by Spinneken and Swan [2009. Second-order wave maker theory using forcefeedback control. Part I. A new theory for regular wave generation. Ocean Engineering, in press, doi:10.1016/j.oceaneng.2009.01.019]. This theory concerns the generation of regular waves by a flap-type wave maker using force-feedback control, providing the first quantitative evidence of the inherent advantages of this latter approach. When the wave maker is controlled by a first-order force command signal, comparisons between the theory and experimental observations confirm two key points: (i) The first-order behaviour is crucial for the absorption characteristics of the machine. (ii) The second-order behaviour leads to a spurious, or unwanted, freely propagating second harmonic that is substantially smaller in amplitude when compared to an identical wave paddle operating with first-order position control. Both aspects of this work, effective absorption and reduced second-order spurious wave generation, are investigated over a broad range of wave frequencies and shown to be widely applicable. Furthermore, the theory also provides a force command signal correct to second order. This is introduced in a separate set of experiments and shown to provide further improvement in the quality of the wave generation.
Spinneken J, Swan C, 2009, Second-order wave maker theory using force-feedback control. Part I. A new theory for regular wave generation, Ocean Engineering, Vol: 36, Pages: 539-548
Second-order wave maker theory has long been established; the most extensive and detailed approach given by Schäffer [1996. Second-order wave maker theory for irregular waves. Ocean Engineering 23, 47–88]. However, all existing theories assume the wave paddle is driven by a position-feedback motion controller. Early research in the wave power field led to the design of a force-controlled absorbing wave machine [Salter, S., 1982. Absorbing wave-makers and wide tanks. In: Directional Wave Spectra Applications, pp. 185–200]. In addition to operating as an excellent absorber, this machine seemed to introduce very little spurious harmonic content when driven with a first-order command signal. The present paper provides a mathematical model for the operation of wave makers using force-feedback control and seeks to explain this apparent advantage. The model is developed to second-order so that a command signal compensating for the remaining spurious wave is also provided. Due to the complexity of the problem, the model has been limited to flap-type wave machines and the generation of regular waves. A variety of numerical tests in force-control mode have been conducted, indicating that the spurious wave content is greatly reduced when compared to the position-control mode. A separate experimental study validating the theory is presented in a part II paper by the same authors.
Spinneken J, Swan C, 2009, Wave generation and absorption using force-controlled wave machines, International Offshore and Polar Engineering Conf., Pages: 397-405
Christou M, Swan C, Gudmestad OT, 2008, The interaction of surface water waves with submerged breakwaters, Coastal Engineering, Vol: 55, Pages: 945-958, ISSN: 0378-3839
Masterton SR, Swan C, 2008, On the accurate and efficient calibration of a 3D wave basin, OCEAN ENGINEERING, Vol: 35, Pages: 763-773, ISSN: 0029-8018
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- Citations: 24
Gibson RS, Swan C, 2007, The evolution of large ocean waves: the role of local and rapid spectral changes, P R SOC A, Vol: 463, Pages: 21-48, ISSN: 1364-5021
This paper concerns the formation of large-focused or near-focused waves in both unidirectional and directional sea-states. When the crests of wave components of varying frequency superimpose at one point in space and time, a large, transient, focused wave can occur. These events are believed to be representative of the largest waves arising in a random sea and, as such, are of importance to the design of marine structures. The details of how such waves form also offer an explanation for the formation of the so-called freak or rogue waves in deep water. The physical mechanisms that govern the evolution of focused waves have been investigated by applying both the fully nonlinear wave model of Bateman et al. (Bateman et al. 2001 J. Comput. Phys. 174, 277 305) and the Zakharov's evolution equation (Zakharov 1968 J. Appl. Mech. Tech. Phys. 9, 190 194). Aspects of these two wave models are complementary, and their combined use allows the full nonlinearity to be considered and, at the same time, provides insights into the dominant physical processes.In unidirectional seas, it has been shown that the local evolution of thewave spectrumleads to larger maximum crest elevations. In contrast, in directional seas, the maximum crest elevation iswellpredictedby a second-order theory based on the underlying spectrum, but the shape of the largest wave is not. The differences between the evolution of large waves in unidirectional and directional sea-states have been investigated by analysing the results of Bateman et al. (2001) using a number of spectral analysis techniques. It has been shown that during the formation of a focused wave event, there are significant and rapid changes to the underlying wave spectrum. These changes alter both the amplitude of the wave components and their dispersive properties. Importantly, in unidirectional sea-states, the bandwidth of the spectrum typically increases; whereas, in directional sea-states it decreases.The changes to thewave spectra have
Christou M, Swan C, Gudmestad OT, 2007, The description of breaking waves and the underlying water particle kinematics, 26th International Conference on Offshore Mechanics and Arctic Engineering, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 291-299
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- Citations: 2
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