The Experts below are selected from a list of 342 Experts worldwide ranked by ideXlab platform
Nobuhito Mori - One of the best experts on this subject based on the ideXlab platform.
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Freak Wave in high order weakly nonlinear Wave evolution with bottom topography change
Coastal Engineering, 2021Co-Authors: Zuorui Lyu, Nobuhito Mori, Hiroaki KashimaAbstract:Abstract In deep-water, quasi-resonant four-Wave interaction gives rise to the occurrence probability of extreme Wave height, which leads to Freak Wave as a natural disaster. As water Wave enters shallow water, four-Wave interaction becomes weak and Wave shoaling makes this effect become complicated. This study conducts a Monte Carlo simulation in modified Nonlinear Schrodinger equation considering bottom topography change, and gives spatial evolution of high-order nonlinearity in Wave train from statistical moment. The result indicates, slope angle plays an important role in a shallow water depth, and steep slope brings about a significant enhancement of extreme event in distribution of maximum Wave height and surface elevation.
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estimation of Freak Wave occurrence from deep to shallow water regions
Coastal Engineering Proceedings, 2014Co-Authors: Hiroaki Kashima, Katsuya Hirayama, Nobuhito MoriAbstract:Nonlinear four-Wave interactions amplify Wave heights of deep-water generating extreme Wave such as a Freak Wave. However, it is not clear the behavior of generated Freak Waves in deep-water shoaling to shallow water regions. In this study, a series of physical experiments and numerical simulations with several bathymetry configurations were conducted for unidirectional random Waves from deep to shallow water regions. The maximum Wave heights increase with an increase in kurtosis by third-order nonlinear interactions in deep water regions. The dependence of the kurtosis on the Freak Wave occurrence is weakened due to second-order nonlinear interactions associated with Wave shoaling on the slope. Moreover, it is possible to understand the behavior of the high-order nonlinearity and the Freak Wave occurrence in shallow water regions if appropriate correction of the insufficient nonlinearity of more than O(e^2) to the standard Boussinesq equation are considered analytically.
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global analysis of Freak Wave based on spectral Wave model
Journal of Japan Society of Civil Engineers, 2012Co-Authors: Nobuhito Mori, Tomohiro Yasuda, Takeru Michimae, Hiroaki Shimada, Hajime MaseAbstract:全球の波浪解析には,デルフト工科大学開発のスペク トル型波浪モデルSWAN(Booji, 1998)を用いた.外力 としてJRA-25(Onogiら, 2004)の海上風U10を与え,全 球を対象に空間解像度1度,周波数36分割,方向分布36 分割の計算条件で波浪推算を行った.解析期間は20032009年の6年間である. Freak Waveに関する波浪諸量の計算に当たり,μ4の推 定にはMoriら(2011)の方向分散角を考慮した推定式を 用いた.μ4からFreak Waveの出現頻度を推定する従来の 予測式では周波数スペクトル幅の影響が考慮されていな いため,実海域に直接適用するには精度不足が問題とな る.そこで,波高分布にWeibull分布を仮定し,その形 状母数にμ4の影響を加えた観測結果を再現できるμ4から Freak Waveの頻度を予測する式(森ら, 2011)を用いて出 現頻度の推定を行った. 波浪推算の精度検証は,全国港湾海洋波浪情報網の波 浪観測データを用いて実施した.解析には,日本海およ び太平洋沿岸合計6観測地点(設置水深50m)を解析対 象として選定し,2004年の1年間の全データについて比 較を行った.
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on the estimation of the kurtosis in directional sea states for Freak Wave forecasting
Journal of Physical Oceanography, 2011Co-Authors: Nobuhito Mori, Miguel Onorato, Peter A. E. M. JanssenAbstract:Based on Monte Carlosimulationsof the nonlinear Schrodingerequationin two horizontal dimensions, the dependence of the kurtosis on the directional energy distribution of the initial conditions is examined. The parametric survey is carried out to obtain the behavior of the kurtosis as function of the Benjamin-Feir index and directional spread in directional sea states. As directional dispersion effect becomes significant, the kurtosis monotonically decreases in comparison with the unidirectional Waves. A parameterization of the kurtosisestimatedfromdirectionalspectra isproposedhere;the erroroftheparameterization isat most10%. The parameterization is verified against laboratory data, and good agreement is obtained.
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Freak Wave AND WEATHER CONDITION
Coastal Engineering Proceedings, 2011Co-Authors: Nobuhito Mori, Hajime Mase, Tomohiro YasudaAbstract:The kurtosis of the surface elevation, Benjamin-Feir Index (BFI) and directional spread are measures of nonlinear four-Wave interactions and Freak Waves. The dependence of kurtosis, BFI and directional spread under typhoon conditions are examined by numerical simulations. The BFI is significantly large in the fourth quadrant of the typhoon while the directional spread is small in the fourth quadrant. It was found that the potentially possible area of Freak Wave occurrence is the fourth quadrant of the typhoon rather than the other quadrants.
Kevin Ewans - One of the best experts on this subject based on the ideXlab platform.
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examining a comprehensive dataset containing thousands of Freak Wave events part 2 analysis and findings
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Marios Christou, Kevin EwansAbstract:This paper concerns the analysis of a very large, quality-controlled dataset of raw Wave measurements. It directly follows from paper 1, as part of work undertaken for the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), and describes the various analyses performed on the dataset. In particular numerous Freak Wave events are observed and various analyses are performed to gain an insight into conditions that are conducive to their formation. The examination of probability distributions, spectral and temporal parameters, degree of focusing and environmental conditions that lead to Freak Waves is performed and the findings are presented.Copyright © 2011 by ASME
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examining a comprehensive dataset containing thousands of Freak Wave events part 1 description of the data and quality control procedure
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Marios Christou, Kevin EwansAbstract:This paper concerns the description of the formation of a very large, quality-controlled dataset of raw Wave measurements. As part of the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), the participants provided raw field measurements of water surface elevation from various installations across the globe. This paper will describe the data collection from the different installations, the strict quality control procedure employed to ensure a reliable dataset, and an overview of the occurrences of records containing Freak Wave events. Part 2 of this paper will then go on to describe the analysis performed on this dataset as well as the findings from the study.Copyright © 2011 by ASME
Ningchuan Zhang - One of the best experts on this subject based on the ideXlab platform.
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experimental study on hydrodynamic characteristics of a moored square cylinder under Freak Wave ii frequency domain study
Ocean Engineering, 2021Co-Authors: Wenbo Pan, Ningchuan Zhang, Chen Liang, Guoxing HuangAbstract:Abstract Previous studies indicated that the occurrence of Freak Wave has a significant impact on the time-domain characteristics of a moored floater dynamic response. In this study, extensive investigations have been performed to figure out further dynamic response characteristics of a moored square cylinder under Freak Wave in frequency-domain. In the experiments, the Wave sequences with and without the Freak Wave are defined as Freak and random Waves, respectively. The results show that, the Freak Wave parameter α1 and spectral peak period have significant impacts on the dynamic response characteristics of the moored floater in frequency-domain. The effect of Freak Wave on surge and mooring tension of the floater is figured out through the low frequency components (0–0.5fp) which leads to significant increase with α1. Followed the occurrence of Freak Wave, surge increases sharply and oscillates for longer than 20 Wave periods in low frequency. Without the occurrence of Freak Wave, however, both amplitude and oscillating of surge is significantly weaker. The mooring tension response is subjected to surge and hence the trend of variation is similar to surge.
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experimental study on motion responses of a moored rectangular cylinder under Freak Waves i time domain study
Ocean Engineering, 2018Co-Authors: Wenbo Pan, Ningchuan Zhang, Guoxing HuangAbstract:Abstract Extensive experiments on the motion responses of a rectangular cylinder under random and Freak Waves have been conducted in the present study. The effects of the relative Wave height, relative period and Freak Wave parameters on the motion responses were investigated. Analysis in time-domain revealed that the Freak Wave parameter α 1 has significant effect on the motion responses of the cylinder, especially for surge and heave. However, α 2 , α 3 , α 4 have not such significant effect on the motion components. With α 1 = 2–2.8, the maximum surge and heave under Freak Waves were 2.5 and 1.5 times, respectively, larger than those under irregular Waves. In addition, the maximum pitch under Freak Waves was approximately 1.3 times of that under irregular Waves. The difference in motion response under Freak Waves and irregular Waves decreased with the increasing relative Wave heights. With Hs/d = 0.03–0.1 and α 1 = 2–2.2, the maximum surge and heave under Freak Waves were approximately 30–60% and 20–40%, respectively, larger than those under irregular Waves. For the effect of the relative period, the critical surge, heave and pitch occur at period around T p /T 0heave ˜ 1.0, 1.5, 2.0 and T p /T 0pitch ˜ 1, respectively. With the natural periods and α 1 = 2–2.2, surge, heave and pitch under Freak Waves were approximately 44%, 40% and 30%, respectively, larger than those under irregular Waves.
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a study on kinematics characteristics of Freak Wave
China Ocean Engineering, 2013Co-Authors: Cheng Cui, Ningchuan Zhang, Shuhua Zuo, Zhuo FangAbstract:Based on the 3rd-order Stokes Wave theory, the speed of Freak Waves is formulated in terms of the period and the Wave height. Finite modified Wave steepness gives rise to a significant enhancement of the nonlinear contributions to the Freak Wave speed in comparison with the 3rd-order Stokes Wave theory. For a fix modified Wave steepness, the estimated amplification of the nonlinear contributions due to the deviation from the 3rd-order Stokes Wave theory is 0.22∼0.99. In addition, the velocity and acceleration fields are also documented in detail. In the present simulation, the horizontal velocities are smaller than the Wave speed, and the Freak Wave exhibits a maximal horizontal velocity up to 37% of the Wave speed and a maximal vertical acceleration up to about 20% of the gravitational acceleration.
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an experimental and numerical study of the Freak Wave speed
Acta Oceanologica Sinica, 2013Co-Authors: Cheng Cui, Ningchuan Zhang, Haigui KangAbstract:The propagation speed is one of the most important characteristics for describing Freak Waves. The research of Freak Wave speed is not only helpful for understanding the generation mechanism and evolution process of Freak Waves, but also applicable to the prediction. A stable and accurate method is proposed for the calculation of the Freak Wave speed, in which physical model tests are carried out to measure the motion of the largest Wave crest along the Wave tank. The linear regression relationship between the spatial position of the largest Wave crest and instantaneous moment is established to calculate the speed of totally 248 cases of experimental Freak Waves and 312 supplementary cases of numerical Freak Waves. Based on the calculate results, a semitheoretical and semiempirical formula is proposed by using a regression analysis method to predict the speed of the Freak Wave, and the nonlinear characteristic of the Freak Wave speed is also investigated.
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numerical study on the effects of uneven bottom topography on Freak Waves
Ocean Engineering, 2012Co-Authors: Cheng Cui, Ningchuan ZhangAbstract:Abstract A numerical model is built by using an improved VOF method coupled with an incompressible Navier–Stokes solver. Exploiting the model, the Freak Wave formation due to the dispersive focusing mechanism is investigated numerically without uneven bottoms and in presence of uneven bottoms. During the Freak Wave transformation over an uneven bottom in finite water, combined effects of shoaling, refraction and reflection can modify the external characteristics of Freak Waves, and also can complicate the energy transfers. Furthermore, Wavelet analysis method is adopted to analyze the behavior of the instantaneous energy structure of Freak Waves. It is found that when the bottoms vary in height, the external characteristic parameters and high frequency energy show a similar trend, but the value may be quite different due to the difference in local characteristic of the bottom.
Marios Christou - One of the best experts on this subject based on the ideXlab platform.
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examining a comprehensive dataset containing thousands of Freak Wave events part 2 analysis and findings
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Marios Christou, Kevin EwansAbstract:This paper concerns the analysis of a very large, quality-controlled dataset of raw Wave measurements. It directly follows from paper 1, as part of work undertaken for the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), and describes the various analyses performed on the dataset. In particular numerous Freak Wave events are observed and various analyses are performed to gain an insight into conditions that are conducive to their formation. The examination of probability distributions, spectral and temporal parameters, degree of focusing and environmental conditions that lead to Freak Waves is performed and the findings are presented.Copyright © 2011 by ASME
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examining a comprehensive dataset containing thousands of Freak Wave events part 1 description of the data and quality control procedure
ASME 2011 30th International Conference on Ocean Offshore and Arctic Engineering, 2011Co-Authors: Marios Christou, Kevin EwansAbstract:This paper concerns the description of the formation of a very large, quality-controlled dataset of raw Wave measurements. As part of the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), the participants provided raw field measurements of water surface elevation from various installations across the globe. This paper will describe the data collection from the different installations, the strict quality control procedure employed to ensure a reliable dataset, and an overview of the occurrences of records containing Freak Wave events. Part 2 of this paper will then go on to describe the analysis performed on this dataset as well as the findings from the study.Copyright © 2011 by ASME
Takuji Waseda - One of the best experts on this subject based on the ideXlab platform.
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temporal variation of modulated Wave train geometries and their influence on vertical bending moments of a container ship
Applied Ocean Research, 2019Co-Authors: Hidetaka Houtani, Takuji Waseda, Katsuji Tanizawa, Hiroshi SawadaAbstract:Abstract A towing experiment was conducted using a modulated Wave train to investigate the vertical bending responses of a hydro-structural container ship model. In the experiment, a spatially periodic modulated Wave train, as a model of a Freak Wave in successive high Waves mimicking the so-called three sisters, was generated by the recently established higher-order spectral method Wave generation (HOSM-WG) method. HOSM-WG enables us to control the location and timing of the maximum crest height in a Wave tank. With precise control of the towing carriage, an experiment was conducted in which the timing of the encounters between the ship model and the modulated Wave train was accurately determined. The maximum sagging moment (SM) was found to increase in proportion with the encounter Wave height. However, because of differences in the relative depth of the fore and aft troughs, the maximum SM is highly variable for a given Wave height. The temporal Wave-geometry evolution caused the relative trough-depth to vary significantly within a Wave period in the vicinity of the maximum crest height. As a result, depending on the encounter timing, the SM varied considerably for a given Wave height. The temporal variation of the Wave geometry is a robust feature of a modulated Wave train and is common between the spatially periodic and temporally periodic modulated Wave trains.
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Impact of the four-Wave quasi-resonance on Freak Wave shapes in the ocean
Ocean Dynamics, 2019Co-Authors: Wataru Fujimoto, Takuji Waseda, Adrean WebbAbstract:Two Freak Waves were observed a day apart in October 2009 at a 5000-m deep moored station in the northwest Pacific Ocean. As the typhoon passed by, the Wave system transitioned within a day from a narrow and unimodal spectrum to the broad and bi-modal spectrum. The occurrence probability of a Freak Wave is known to increase due to a modulational instability; however, whether the modulational instability survives under a realistic directional sea state has not been conclusively determined yet. In this study, a phase-resolving Wave model was used to obtain ensembles of realizations based on observed and simulated directional spectra. Unlike previous studies that focused only on the probability of Freak Wave occurrence, this study focuses on Wave shape. It reveals that the front-to-rear asymmetry and crescent shape deformation of the crest are more pronounced for narrower spectrum and longer-lifetime Freak Waves; this distortion of Wave shape and extended lifetime are both characteristics of nonlinear Wave groups. This study also shows that the distribution of the lifetime of a Freak Wave depends on the sea state and that the number of nonlinear Wave groups increases for a narrower spectrum. We therefore conjecture that both the four-Wave quasi-resonance and dispersive focusing are responsible for Freak Wave generation, but their relative significance depends on the spectral broadness. Investigating the total kurtosis or occurrence probability alone is insufficient to unravel the underlying mechanisms of individual Freak-Wave generation.
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Freak Wave generation in a Wave basin with hosm wg method
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE, 2015Co-Authors: Hidetaka Houtani, Takuji Waseda, Wataru Fujimoto, Keiji Kiyomatsu, Katsuji TanizawaAbstract:A method to produce Freak Waves with arbitrary spectrum in a fully directional Wave basin is presented here. This is an extension of Waseda, Houtani and Tanizawa at OMAE 2013[1], which used “HOSM-WG” based on the higher-order spectral method (HOSM). We used the following three methods to improve the HOSM-WG in [1]: “separation of free Waves from bound Waves,” “using Biesel’s transfer function in Wavenumber space” and “using Schaffer’s 2nd-order Wave maker control method.” Modulational Wave trains, Freak Waves in unidirectional irregular Waves and Freak Waves in short-crested irregular Waves were generated in a Wave basin. The experimental results using the improved HOSM-WG were compared to the HOSM simulation, and good agreements were found. The effectiveness of the improved HOSM-WG was ascertained. We showed that the difference between HOSM-WG and HOSM simulations became larger as Wave steepness, frequency bandwidth of the spectrum or directional spreading became larger.Copyright © 2015 by ASME
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predicting Freakish sea state with an operational third generation Wave model
Natural Hazards and Earth System Sciences, 2014Co-Authors: Takuji Waseda, Hitoshi Tamura, Keiji Kiyomatsu, Yasumasa Miyazawa, K IyamaAbstract:Abstract. The understanding of Freak Wave generation mechanisms has advanced and the community has reached a consensus that spectral geometry plays an important role. Numerous marine accident cases were studied and revealed that the narrowing of the directional spectrum is a good indicator of dangerous sea. However, the estimation of the directional spectrum depends on the performance of the third-generation Wave model. In this work, a well-studied marine accident case in Japan in 1980 (Onomichi-Maru incident) is revisited and the sea states are hindcasted using both the DIA (discrete interaction approximation) and SRIAM (Simplified Research Institute of Applied Mechanics) nonlinear source terms. The result indicates that the temporal evolution of the basic parameters (directional spreading and frequency bandwidth) agree reasonably well between the two schemes and therefore the most commonly used DIA method is qualitatively sufficient to predict Freakish sea state. The analyses revealed that in the case of Onomichi-Maru, a moving gale system caused the spectrum to grow in energy with limited downshifting at the accident's site. This conclusion contradicts the marine inquiry report speculating that the two swell systems crossed at the accident's site. The unimodal Wave system grew under strong influence of local wind with a peculiar energy transfer.
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enhanced Freak Wave occurrence with narrow directional spectrum in the north sea
Geophysical Research Letters, 2011Co-Authors: Takuji Waseda, M Hallerstig, K Ozaki, H TomitaAbstract:[1] Wind and Wave records obtained from the Kvitebjorn platform (2.3°E, 61.0°N, 190 m deep) in the northern North Sea from 2003 to 2005 were analyzed. Among the 2723 20-min records taken during storm conditions, 57 cases included Freak Waves exceeding twice the significant Wave height. Comparisons between various Wave parameters and the Freak Wave occurrence index did not show any significant correlation. Thus, the in situ Wave record was used to select the days when relatively more or less Freak Waves were observed. The days were classified into Freakish and non-Freakish days, respectively. On Freakish days, the Icelandic low was enhanced. Hindcasts performed by a third-generation Wave model suggest that this synoptic atmospheric pressure difference produces approximately 7.6 degrees narrower directional spreading of the Wave spectra during Freakish days at the Kvitebjorn platform. This result is consistent with the physical mechanism of Freak Wave generation through nonlinear self-focusing in random Wave fields.
