Laboratory measurements of limiting freak waves on currents

Coastal Engineering Proceedings, 2014

Physical and numerical modeling of initially monochromatic wave propagation on stepwise opposite current in conditions of deep water was carrying out. The physical experiments demonstrated two steps of downshifting of spectral maximum in the regions of increasing of opposite current. Freak waves arise at the moments of downshifting, when several peaks of spectrum exist, due to superposition of waves, provided by each of spectral peaks. To explain the phenomena of downshifting the dynamical evolution set of equations are constructed. Their solutions perfectly described the main qualitative features of wave transformation during the physical experiments. Keywords: waves on opposite current; frequency downshifting; freak waves Experiment was carried out in middle size flume of Tainan Hydraulics Laboratory: 200 m long, 2 m wide and 2 m deep. Setup of experiment is shown on Fig. . A current was generated by pump. A space variety of current was provided by decreasing of water depth above underwater bar, placed in

Coastal Engineering Proceedings, 2012

We investigate the dynamic and kinematic characteristics of freak waves using a direct phase-resolved nonlinear numerical method. The focus is on the understanding of the effects of different nonlinear wave-wave interactions on freak waves development and characteristics in the evolution process of modulated Stokes wave trains. Long time simulations of modulated Stokes wave trains, with different parameters, are obtained. Based on these simulations, we find that there are different kinds of freak waves in different time scales due to two kinds of different nonlinear mechanisms. One is the modulation instability and another related to the wave group interaction. Both the dynamic and kinematic characteristics of the different kinds of freak waves are distinct. Occurrence of freak waves (especially of large height) is usually correlated with broadband wave spectra.

Four focusing models for generation of freak waves are presented. An extreme wave focusing model is presented on basis of the enhanced High2Order Spectral (HOS) method and the importance of the nonlinear wave2wave interaction evaluated by comparison of the calculated results with experimental and theoretical data. Based on the modification of Longuet2Higgins model , four wave models for generation of freak waves (a. extreme wave model + random wave model ; b. extreme wave model + regular wave model ; c. phase interval modulation wave focusing model ; d. number modulation wave focusing model with the same phase) are proposed. By use of different energy distribution techniques in four models , freak wave events are obtained with different Hmax/ Hs in finite space and time.

Journal of Fluid Mechanics, 2009

Linear refraction of waves on inhomogeneous current is known to provoke extreme waves. We investigate the effect of nonlinearity on this phenomenon, with respect to the variation of significant wave height, kurtosis and occurrence of freak waves. Monte Carlo simulations are performed employing a modified nonlinear Schrödinger equation that includes the effects of a prescribed non-potential current. We recommend that freak waves should be defined by a local criterion according to the wave distribution at each location of constant current, not by a global criterion that is either averaged over, or insensitive to, inhomogeneities of the current. Nonlinearity can reduce the modulation of significant wave height. Depending on the configuration of current and waves, the kurtosis and probability of freak waves can either grow or decrease when the wave height increases due to linear refraction. At the centre of an opposing current jet where waves are known to become large, we find that frea...

Physica D: Nonlinear Phenomena, 2000

The mechanism of the freak wave formation related to the spatial-temporal focusing is studied within the framework of the Korteweg-de Vries equation. A method to find the wave trains whose evolution leads to the freak wave formation is proposed. It is based on the solution of the Korteweg-de Vries equation with an initial condition corresponding to the expected freak wave. All solutions of this Cauchy problem by the reversal of abscissa represent the possible forms of wave trains which evolve into the freak wave. It is found that freak waves are almost linear waves, and their characteristic Ursell parameter is small. The freak wave formation is possible also from the random wave field and the numerical simulation describes the details of this phenomenon. It is shown that freak waves can be generated not only for specific conditions, but also for relative wide classes of the wave trains. This mechanism explains the rare and short-lived character of the freak wave.

European Journal of Mechanics - B/Fluids, 2006

The freak wave formation due to the dispersive focusing mechanism is investigated experimentally without wind and in presence of wind. An asymmetric behaviour between the focusing and defocusing stages is found when the wind is blowing over the mechanically generated gravity wave group. This feature corresponds physically to the sustain of the freak wave mechanism on longer periods of time. Furthermore, a weak amplification of the freak wave and a shift in the downstream direction of the point where the waves merge are observed. The experimental results suggest that the Jeffreys' sheltering mechanism could play a key role in the coherence of the group of the freak wave. Hence, the Jeffreys' sheltering theory is introduced in a fully nonlinear model. The results of the numerical simulations confirm that the duration of the freak wave event increases with the wind velocity.

Journal of Physics: Conference Series

This paper describes freak waves by a (pseudo-)maximal wave proposed in [1]. The freak wave is a consequence of a group event that is present in a time signal at some position and contains successive high amplitudes with different frequencies. The linear theory predicts the position and time of the maximal amplitude wave quite well by minimizing the variance of the total wave phase of the given initial signal. The formation of the freak wave is shown to be mainly triggered by the local interaction of waves evolving from the group event that already contained large local energy. In the evolution, the phases become more coherent and the local energy is focussed to develop a larger amplitude. We investigate two laboratory experimental signals, a dispersive focussing wave with harmonic background and a scaled New Year wave. Both signals generate a freak wave at the predicted position and time and the freak wave can be described by a pseudo-maximal wave with specific parameters. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Engineering Sciences, 2021

Control signals with simultaneous modulation of periods and amplitudes were finetuned and fed to a wave flap for a generation of freak waves. The meshless Smoothed Particle Hydrodynamics method was used to predict the location and the amplitude of the maximum wave crest. The time series output from the DualSPHysics software was validated experimentally at the BSHC seakeeping wave basin. The experimental data is in good agreement with the simulations.

Physical Review Letters, 2010

Microwave transport experiments have been performed in a quasi-two-dimensional resonator with randomly distributed conical scatterers. At high frequencies, the flow shows branching structures similar to those observed in stationary imaging of electron flow. Semiclassical simulations confirm that caustics in the ray dynamics are responsible for these structures. At lower frequencies, large deviations from Rayleigh's law for the wave height distribution are observed, which can only partially be described by existing multiple-scattering theories. In particular there are "hot spots" with intensities far beyond those expected in a random wave field. The results are analogous to flow patterns observed in the ocean in the presence of spatially varying currents or depth variations in the sea floor, where branches and hot spots lead to an enhanced frequency of freak or rogue wave formation.

2004

The occurrence probability of freak waves is formulated as a function of number of waves and surface elevation kurtosis based on the weakly non-Gaussian theory. Finite kurtosis gives rise to a significant enhancement of freak wave generation. For fixed number of waves, the estimated amplification ratio of freak wave occurrence due to the deviation from the Gaussian theory is 50%-300%.