ORCID Profile
0000-0001-5403-5303
Current Organisation
University of Tokyo
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Publisher: American Geophysical Union (AGU)
Date: 03-2010
DOI: 10.1029/2009GL041771
Publisher: Elsevier BV
Date: 08-2019
Publisher: Cambridge University Press (CUP)
Date: 16-03-2015
DOI: 10.1017/JFM.2015.132
Abstract: Interaction with an opposing current lifies wave modulation and accelerates nonlinear wave focusing in regular wavepackets. This results in large- litude waves, usually known as rogue waves, even if the wave conditions are less prone to extremes. Laboratory experiments in three independent facilities are presented here to assess the role of opposing currents in changing the statistical properties of unidirectional and directional mechanically generated random wavefields. The results demonstrate in a consistent and robust manner that opposing currents induce a sharp and rapid transition from weakly to strongly non-Gaussian properties. This is associated with a substantial increase in the probability of occurrence of rogue waves for unidirectional and directional sea states, for which the occurrence of extreme and rogue waves is normally the least expected.
Publisher: Springer Science and Business Media LLC
Date: 14-03-2018
DOI: 10.1038/S41598-018-22500-9
Abstract: The long-term trend of extreme ocean waves in the emerging ice-free waters of the summer Arctic is studied using ERA-Interim wave reanalysis, with validation by two drifting wave buoys deployed in summer 2016. The 38-year-long reanalysis dataset reveals an increase in the expected largest significant wave height from 2.3 m to 3.1 m in the ice-free water from the Laptev to the Beaufort Seas during October. The trend is highly correlated with the expected increase in highest wind speed from 12.0 m/s to 14.2 m/s over the ice-free ocean, and less so with the extent of the ice-free water. Since the storms in this area did not strengthen throughout the analysis period, the increase in the expected largest significant wave height follows from the enhanced probability of storms in ice-free waters, which is pertinent to the estimation of extreme sea conditions along the Northern Sea Route.
Publisher: The Royal Society
Date: 12-09-2022
Abstract: Waves in the Marginal Ice Zone in the Okhotsk Sea are less studied compared to the Antarctic and Arctic. In February 2020, wave observations were conducted for the first time in the Okhotsk Sea, during the observational program by Patrol Vessel Soya. A wave buoy was deployed on the ice, and
Publisher: Elsevier BV
Date: 03-2021
Publisher: American Physical Society (APS)
Date: 15-07-2020
Publisher: American Physical Society (APS)
Date: 29-05-2013
Publisher: American Meteorological Society
Date: 09-2016
Abstract: Laboratory experiments were performed to investigate the effects of a coflowing current field on the spectral shape of water waves. The results indicate that refraction is the main factor in modulating wave height and overall wave energy. Although the structure of the current field varies considerably, some current-induced patterns in the wave spectrum are observed. In high frequencies, the energy cascading generated by nonlinear interactions is suppressed, and the development of a spectral tail is disturbed, as a consequence of the detuning of the four-wave resonance conditions. Furthermore, the presence of currents slows the downshifting of the spectral peak. The suppression of the high-frequency energy under the influence of currents is more prominent as the spectral steepness increases. The energy suppression is also more accentuated and long-standing along the fetch when the directional spreading of waves is sufficiently broad. This result indicates that the current-induced detuning of resonant conditions is more effective when exact resonances are the primary mechanism of nonlinear interactions than when quasi resonances prevail (directionally narrow cases). Additionally, the directional analysis shows that the highly variable currents broaden the directional spreading of waves. The broadening is suggested to be related to random refraction and scattering of wave rays. The random disturbance of wavenumbers alters the nonlinear interaction conditions and weakens the energy exchanges among wave components, which is expressed in the suppression of the high-frequency energy.
Publisher: American Physical Society (APS)
Date: 20-03-2009
Publisher: American Physical Society (APS)
Date: 09-11-2020
Publisher: American Meteorological Society
Date: 2011
Abstract: Wave breaking is observed in a laboratory experiment with waves of realistic average steepness and directional spread. It is shown that a modulational-instability mechanism is active in such circumstances and can lead to the breaking. Experiments were conducted in the directional wave tank of the University of Tokyo, and the mechanically generated wave fields consisted of a primary wave with sidebands in the frequency domain, with continuous directional distribution in the angular domain. Initial steepness of the primary wave and sidebands, as well as the width of directional distributions varied in a broad range to determine the combination of steepness/directional-spread properties that separates modulational-instability breaking from the linear-focusing breaking.
Publisher: Elsevier BV
Date: 12-2018
Publisher: Springer Science and Business Media LLC
Date: 31-08-2017
Publisher: Springer Science and Business Media LLC
Date: 18-07-2018
Publisher: IEEE
Date: 07-2019
Publisher: MDPI AG
Date: 20-05-2022
DOI: 10.3390/JMSE10050697
Abstract: The occurrence of exceptionally large waves in regions of high maritime traffic has severe consequences, ranging from complicating navigation routes to the loss of ships and human lives [...]
Publisher: Proceedings of the National Academy of Sciences
Date: 26-04-2019
Abstract: Understanding the fundamental dynamics of directional and localized waves is of significant importance for modeling ocean waves as well as predicting extreme events. We report a theoretical framework, based on the universal (2D + 1) nonlinear Schrödinger equation, that allows the construction of slanted solitons and breathers on the water surface. Our corresponding wave flume observations emphasize and uniquely reveal that short-crested localizations can be described as a result of nonlinear wave dynamics, complementing the linear superposition and interference arguments as has been generally suggested for directional ocean waves.
Publisher: Frontiers Media SA
Date: 09-07-2019
Publisher: AIP Publishing
Date: 09-2013
DOI: 10.1063/1.4821810
Abstract: We present a laboratory experiment in a large directional wave basin to discuss the instability of a plane wave to oblique side band perturbations in finite water depth. Experimental observations, with the support of numerical simulations, confirm that a carrier wave becomes modulationally unstable even for relative water depths k0h & 1.36 (with k the wavenumber of the plane wave and h the water depth), when it is perturbed by appropriate oblique disturbances. Results corroborate that the underlying mechanism is still a plausible explanation for the generation of rogue waves in finite water depth.
Publisher: MDPI AG
Date: 02-05-2019
Abstract: Amplitude modulation of a propagating wave train has been observed in various media including hydrodynamics and optical fibers. The notable difference of the propagating wave trains in these media is the magnitude of the nonlinearity and the associated spectral bandwidth. The nonlinearity and dispersion parameters of optical fibers are two orders of magnitude smaller than the hydrodynamic counterparts, and therefore, considered to better assure the slowly varying envelope approximation (SVEA) of the nonlinear Schrödinger equations (NLSE). While most optics experiment demonstrate an NLSE-like symmetric solutions, experimental studies by Dudley et al. (Optics Express, 2009, 17, 21497–21508) show an asymmetric spectral evolution in the dynamics of unstable electromagnetic waves with high intensities. Motivated by this result, the hydrodynamic Euler equation is numerically solved to study the long-term evolution of a water-wave modulated wave train in the optical regime, i.e., at small steepness and spectral bandwidth. As the initial steepness is increased, retaining the initial spectral bandwidth thereby increasing the Benjamin–Feir Index, the modulation localizes, and the asymmetric and broad spectrum appears. While the deviation of the evolution from the NLSE solution is a result of broadband dynamics of free wave interaction, the resulting asymmetry of the spectrum is a consequence of the violation of the SVEA.
Publisher: Cambridge University Press (CUP)
Date: 29-10-2015
DOI: 10.1017/JFM.2015.578
Abstract: A series of experiments were conducted in a wave basin (50 m long, 10 m wide and 5 m deep) generating two waves propagating at an angle by a directional wavemaker. When the two waves were selected from a resonant triplet, an initially non-existing wave grew as the waves propagated down the tank. The linear growth rate of the resonating wave agreed well with third-order resonance theory based on Zakharov’s reduced gravity equation. Additional experiments with opposing and coflowing mean current with large temporal and spatial variations were conducted. As the flow rate increased, the linear growth was suppressed. As reproduced numerically with Zakharov’s equation, the resonant interaction saturated at time scales inversely proportional to the magnitude of the forced random resonance detuning. It is conjectured that the resonance is detuned by the variation and not by the mean of the current field due to wavelength-dependent Doppler shift and to the refraction of wave rays. Further analysis of the spectral evolution revealed that while discrete peaks appear at high frequencies as a result of dynamical cascading, a continuously saturated spectrum develops in the background as the current speed increases. Additional experiments were conducted studying the evolution of the random directional wave on a dynamical time scale under the influence of current. Due to random resonance detuning by the current, the spectral tail tended to be suppressed.
Location: United States of America
No related grants have been discovered for Takuji Waseda.