35 results found
Ewans K, Christou M, Ilic S, et al., 2020, Identifying Higher-order Interactions in Wave Time-series, Journal of Offshore Mechanics and Arctic Engineering: Transactions of the ASME, ISSN: 0195-0738
Craciunescu C, Christou M, 2020, Wave breaking energy dissipation in long-crested focused wave groups based on JONSWAP spectra, Applied Ocean Research, Vol: 99, ISSN: 0141-1187
The present study is a new experimental investigation into energy dissipation due to wave breaking. Long-crested wave groups based on JONSWAP spectra are generated in a wave flume to estimate the energy loss from the wave packet due to the occurrence of single breaking events. These events range from gentle spilling to large plunging breakers. The fractional energy losses and breaking strength parameter are shown to be in good agreement with previous studies based on other spectral shapes. Due to the significance of wave breaking in the evolution of large ocean waves, the dissipative model of Tian et al. (2010)  is examined and improved upon for JONSWAP spectra. Local wave geometry and breaking duration obtained from measurements using an accurate two-camera setup are employed to obtain new parameterisations, directly applicable to realistic ocean waves. An increase of approximately 85% in the proportionality constant needed to compute νeddy is deemed necessary such that energy losses from the new experimental wave groups are accurately captured. Furthermore, it is shown that the time and length scales of the breaking process are well correlated to the local steepness of the wave event, and new fits are obtained that considerably reduce the scatter of the original model. As focused wave groups represent the most probable shape of large wave events in the open seas, the improved model can be implemented in computationally-efficient numerical potential flow solvers to simulate energy dissipation from breaking events in realistic waves.
Craciunescu CC, Christou M, 2020, On the calculation of wavenumber from measured time traces, Applied Ocean Research, Vol: 98, Pages: 1-16, ISSN: 0141-1187
When characterising local geometry for unsteady water waves, the accurate determination of spatial quantities such as the wavenumber from measured time traces is non-trivial. The present study investigates different methods in order to highlight the discrepancies that arise when estimating the wavenumber from fully nonlinear numerical time traces of the free surface profile. To this extent, the open research topic relating to the accuracy of the Hilbert transform in analysing broad-banded signals is addressed. The accuracy of local wavenumber estimates from the Hilbert transform are shown to have a strong dependency on the bandwidth of the underlying spectrum. For broad-banded sea-states there is a significant departure of the Hilbert transform predictions from actual wavenumber values. In contrast, the Double Fourier numerical model of Baldock and Swan (1994)  is shown to obtain accurate estimates of local wave geometry from just a single measured time trace, regardless of spectral characteristics; it is thus the recommended method. In addition, an empirical correction factor is proposed that can be applied to the linear dispersion equation in order to obtain an improved wavenumber estimate. The correction is tested for unidirectional deep water, nonlinear random sea-states and shown to offer considerable improvements in the local wavenumber estimates.
Karmpadakis I, Swan C, Christou M, 2020, Assessment of wave height distributions using an extensive field database, Coastal Engineering, Vol: 157, Pages: 1-15, ISSN: 0378-3839
The present paper investigates the short-term statistical distribution of wave heights. Specifically, some of the most commonly applied wave height distributions are assessed using field measurements. The latter comprise of wave radar observations from 10 different locations in the North Sea and cover water depths between 7.7 m and 45 m. In total, the field database includes more than 200 million waves, making it one of the largest of its kind in this water depth regime. In using these data, the accuracy of existing wave height distributions has been examined and guidance is provided concerning the best performing models and their domain of applicability. Additionally, insights concerning the influence of key met-ocean parameters are also provided. Taken together, the results in this paper present an overview of the statistical behaviour of wave heights in finite water depths, as observed in the field.
Eichentopf S, Alsina J, Christou M, et al., 2020, Storm sequencing and beach profile variability at Hasaki, Japan, Marine Geology, ISSN: 0025-3227
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.
Craciunescu CC, Christou M, 2019, The effect of wave crest speed in breaking waves, Publisher: IOP Publishing, Pages: 012039-012039
Karmpadakis I, Swan C, Christou M, 2019, Wave height and crest height distributions in shallow water — Analysis of field data, Coastal Structures
Ewans K, Christou M, Ilic S, et al., 2019, Identifying Higher-Order Interactions in Wave Time-Series, ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: American Society of Mechanical Engineers
<jats:title>Abstract</jats:title> <jats:p>Reliable design and reanalysis of coastal and offshore structures requires, amongst other things, characterisation of extreme crest elevation corresponding to long return periods, and of the evolution of a wave in space and time conditional on an extreme crest.</jats:p> <jats:p>Extreme crests typically correspond to focussed wave events enhanced by wave-wave interactions of different orders. Higher-order spectral analysis can be used to identify wave-wave interactions in time-series of water surface elevation.</jats:p> <jats:p>The bispectrum and its normalised form (the bicoherence) have been reported by numerous authors as a means to characterise three-wave interactions in laboratory, field and simulation experiments. The bispectrum corresponds to a frequency-domain representation of the third order cumulant of the time-series, and can be thought of as an extension of the power spectrum (itself the frequency-domain representation of the second order cumulant). The power spectrum and bispectrum can both be expressed in terms of the Fourier transforms of the original time-series. The Fast Fourier transform (FFT) therefore provides an efficient means of estimation. However, there are a number of important practical considerations to ensuring reasonable estimation.</jats:p> <jats:p>To detect four-wave interactions, we need to consider the trispectrum and its normalised form (the tricoherence). The trispectrum corresponds to a frequency-domain (Fourier) representation of the fourth-order cumulant of the time-series. Four-wave interactions between Fourier components can involve interactions of the type where f1 + f2 + f3 = f4 and where f1 + f2 = f3 + f4, resulting in two definitions of the trispectrum, depending on which of the two interactions is of interest. We consider both definitions in this paper. Both definitions c
Gibson R, Christou M, 2019, Metocean Criteria for the Fatigue Analysis of Subsea Pipelines, ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering, Publisher: American Society of Mechanical Engineers
<jats:title>Abstract</jats:title> <jats:p>This paper is concerned with a methodology for defining metocean criteria for the analysis of subsea pipelines. The approach that has been developed incorporates the statistical dependence between wave and current loading. The method achieves this through a combination of the application of the conditional extreme value model of Heffernan and Tawn (2004) and clustering algorithms. This is particularly important in regions where the currents are largely wind driven as there can be a strong correlation between the two. The results can be used to assess fatigue damage from vortex-induced-vibrations and wave loading. The same model can be used to derive deterministic design events. The method is applied to a pipeline in the Caspian Sea. The results are compared to models that assume either independence or a perfect correlation between waves and currents. It is shown that simpler models that do not accurately model the dependence between the waves and the currents can provide estimates of fatigue damage that are an order of magnitude too high or too low.</jats:p>
Craciunescu C, Christou M, 2019, Identifying Breaking Waves from Measured Time Traces, 29th International Ocean and Polar Engineering Conference
Coffey C, Gibson R, Christou M, 2019, Uncertainty in explosion risk assessment, 29th Hazards Conference
Angeloudis A, Piggott MD, Kramer SC, et al., 2017, Comparison of 0-D, 1-D and 2-D model capabilities for tidal range energy resource assessments, European Wave and Tidal Energy Conference, Cork, Ireland
Beyer M, Swan C, Christou M, 2017, Focused waves on depth-varying currents: the role of vorticity, 27th International Ocean and Polar Engineering Conference
Hadjigeorgiou D, Swan C, Christou M, 2017, Numerical investigation of crest-height statistics in deep water, 27th International Ocean and Polar Engineering Conference
Makri I, Rose S, Christou M, et al., 2016, Examining field measurements of deep-water crest statistics, Conference of Ocean, Offshore and Arctic Engineering
Jaouen F, Waals O, de Jong M, et al., 2016, Methodology for the design of LNG terminals in a nearshore environment, 35th Conference of Ocean, Offshore and Arctic Engineering
Wu Y, Randell D, Christou M, et al., 2016, On the distribution of wave height in shallow water, Coastal Engineering, Vol: 111, Pages: 39-49, ISSN: 0378-3839
The statistical distribution of the height of sea waves in deep water has been modelled using the Rayleigh (Longuet-Higgins, 1952) and Weibull distributions (Forristall, 1978). Depth-induced wave breaking leading to restriction on the ratio of wave height to water depth requires new parameterisations of these or other distributional forms for shallow water. Glukhovskiy (1966) proposed a Weibull parameterisation accommodating depth-limited breaking, modified by van Vledder (1991). Battjes and Groenendijk (2000) suggested a two-part Weibull–Weibull distribution. Here we propose a two-part Weibull-generalised Pareto model for wave height in shallow water, parameterised empirically in terms of sea state parameters (significant wave height, HS, local wave-number, kL, and water depth, d), using data from both laboratory and field measurements from 4 offshore locations. We are particularly concerned that the model can be applied usefully in a straightforward manner; given three pre-specified universal parameters, the model further requires values for sea state significant wave height and wave number, and water depth so that it can be applied. The model has continuous probability density, smooth cumulative distribution function, incorporates the Miche upper limit for wave heights (Miche, 1944) and adopts HS as the transition wave height from Weibull body to generalised Pareto tail forms. Accordingly, the model is effectively a new form for the breaking wave height distribution. The estimated model provides good predictive performance on laboratory and field data.
Gibson R, Christou M, Feld G, 2014, The statistics of wave height and crest elevation during the December 2012 storm in the North Sea, OCEAN DYNAMICS, Vol: 64, Pages: 1305-1317, ISSN: 1616-7341
Christou M, Ewans K, 2014, Field Measurements of Rogue Water Waves, JOURNAL OF PHYSICAL OCEANOGRAPHY, Vol: 44, Pages: 2317-2335, ISSN: 0022-3670
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.
Gibson R, Christou M, Feld G, 2013, EXAMINING FIELD MEASUREMENTS DURING THE DECEMBER 2012 STORM IN THE NORTH SEA, PROCEEDINGS OF THE 13TH INTERNATIONAL WORKSHOP ON WAVE HINDCASTING & FORECASTING & 4TH COASTAL HAZARDS SYMPOSIUM, Banff, Canada
Maris J, Christou M, Huijsmans R, 2012, Investigating the Use of the Hilbert-Huang Transform for the Analysis of Freak Waves, PROCEEDINGS OF THE 31ST CONFERENCE OF OFFSHORE MECHANICS & ARCTIC ENGINEERING
Christou M, Ewans K, 2011, Examining a comprehensive dataset with thousands of freak wave events. Part 1 - Description of the data and the quality control procedure, PROCEEDINGS OF THE 30TH CONFERENCE OF OFFSHORE MECHANICS & ARCTIC ENGINEERING
Buchner B, Forristall G, Ewans K, et al., 2011, New insights in extreme crest height distributions - A summary of the CresT JIP, PROCEEDINGS OF THE 30TH CONFERENCE OF OFFSHORE MECHANICS & ARCTIC ENGINEERING
Christou M, Rijnsdorp DP, Ewans K, 2011, Analysis of shallow water wave measurements recorded at the Field Research Facility, PROCEEDINGS OF THE 12TH INTERNATIONAL WORKSHOP ON WAVE HINDCASTING & FORECASTING & 3RD COASTAL HAZARDS SYMPOSIUM
Christou M, Ewans K, 2011, Examining a comprehensive dataset with thousands of freak wave events. Part 2 - Analysis and findings, PROCEEDINGS OF THE 30TH CONFERENCE OF OFFSHORE MECHANICS & ARCTIC ENGINEERING
Christou M, Ewans K, Clauss GF, et al., 2010, The spatial evolution of the spectral characteristics of the Draupner new year wave, 29TH CONFERENCE OF OFFSHORE MECHANICS & ARCTIC ENGINEERING
Christou M, 2009, Fully Nonlinear Computations of Waves and Wave-structure Interaction
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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