Qualitative comparisons of experimental results on deterministic freak wave generation based on modulational instability

Abstract In this study, probability of freak wave occurrence due to modulational instability in JONSWAP sea states are investigated. This investigation has been conducted based on the quantitative indicators of instability in wave spectrum, which are two Benjamin-Feir index (BFI) [1,2] with different spectral bandwidth definitions and Π number [3]. Evolution of wave field are simulated using fully nonlinear phase-resolving Chalikov-Sheinin (CS) numerical model [4,5]. Initial sea surface is controlled with JONSWAP shape parameters (α and γ) and random initial phases. Effect of high frequency end of spectrum on modulational instability and freak wave evolution are discussed by considering 4 different tail lengths. According to simulation results, all parameters that are considered here perform as an indicator for the occurrence of extreme events which makes it possible to define a certain interval for indicators, where freak wave occurrence probability is the highest and potentially dangerous, to be possibly used in extreme wave forecasting. Another key finding is that, modulational instability increases when high frequency part of spectrum is present (longer tail) as expected. Nevertheless, after certain nonlinearity, modulational instability is more prone to result in breaking which significantly decreases the probability of occurrence of freak events. Therefore, spectra with shorter tail length result in more dangerous sea states.

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Experimental investigation on the evolution of the modulation instability with dissipation

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.372 ◽

2012 ◽

Vol 711 ◽

pp. 101-121 ◽

Cited By ~ 13

Author(s):

Y. Ma ◽

G. Dong ◽

M. Perlin ◽

X. Ma ◽

G. Wang

Keyword(s):

Experimental Investigation ◽

Modulational Instability ◽

Experimental Results ◽

Damping Term ◽

Wave Flume ◽

Perturbation Frequency ◽

Linear Damping ◽

Wave Trains ◽

Momentum Integral ◽

The Waves

AbstractAn experimental investigation focusing on the effect of dissipation on the evolution of the Benjamin–Feir instability is reported. A series of wave trains with added sidebands, and varying initial steepness, perturbed amplitudes and frequencies, are physically generated in a long wave flume. The experimental results directly confirm the stabilization theory of Segur et al. (J. Fluid Mech., vol. 539, 2005, pp. 229–271), i.e. dissipation can stabilize the Benjamin–Feir instability. Furthermore, the experiments reveal that the effect of dissipation on modulational instability depends strongly on the perturbation frequency. It is found that the effect of dissipation on the growth rates of the sidebands for the waves with higher perturbation frequencies is more evident than on those of waves with lower perturbation frequencies. In addition, numerical simulations based on Dysthe’s equation with a linear damping term included, which is estimated from the experimental data, can predict the experimental results well if the momentum integral of the wave trains is conserved during evolution.

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Three reasons for freak wave generation in the non-uniform current

European Journal of Mechanics - B/Fluids ◽

10.1016/j.euromechflu.2006.02.009 ◽

2006 ◽

Vol 25 (5) ◽

pp. 574-585 ◽

Cited By ~ 34

Author(s):

I.V. Lavrenov ◽

A.V. Porubov

Keyword(s):

Wave Generation ◽

Freak Wave ◽

Uniform Current

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Research on the Time-Frequency Energy Structure of Freak Wave Generation and Evolution

Volume 7: CFD and VIV; Offshore Geotechnics ◽

10.1115/omae2011-49292 ◽

2011 ◽

Cited By ~ 1

Author(s):

Cheng Cui ◽

Ning Chuan Zhang

Keyword(s):

Wave Height ◽

Characteristic Parameter ◽

Wave Generation ◽

Energy Concentration ◽

Energy Structure ◽

Superposition Model ◽

Transient Wave ◽

Large Wave ◽

Freak Wave ◽

Time Frequency

In this paper, simulations of freak wave generation and evolution are carried out in a 2-D Numerical Wave Tank (NWT), which is based on the improved Volume of fluid method (VOF). The freak wave can be generated by the individual superposition model or the dual superposition model (a transient wave is embedded into a random wave train). Wavelet analysis method has been adopted to calculate the time-frequency energy spectrums of the wave trains collected during freak wave generation and evolution. In the generation course of the freak wave, the energy is focused gradually and the energy is gradually diffused in the course of evolution. When the most appropriate energy concentration occurs, the freak wave results. The moderately large wave, which occurs during the period around the freak wave and does not satisfy all the qualifications of the adopted definition, has a similar time-frequency energy structure with the freak wave. In its wavelet spectrum, strong energy density surges instantaneously and seemingly carried over to the high frequency components at the instant the large wave occurs. The external characteristic parameter α1 (the ratio of the maximum wave height to the significant wave height) and the internal characteristic parameter αE (energy concentration) appear to correspond.

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Wave groupiness analysis of the process of 2D freak wave generation in random wave trains

Ocean Engineering ◽

10.1016/j.oceaneng.2015.05.034 ◽

2015 ◽

Vol 104 ◽

pp. 480-488 ◽

Cited By ~ 10

Author(s):

Jinxuan Li ◽

Jiqing Yang ◽

Shuxue Liu ◽

Xinran Ji

Keyword(s):

Wave Generation ◽

Random Wave ◽

Freak Wave ◽

Wave Trains ◽

Wave Groupiness

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Influence of wind on extreme wave events: experimental and numerical approaches

Journal of Fluid Mechanics ◽

10.1017/s0022112007009019 ◽

2007 ◽

Vol 594 ◽

pp. 209-247 ◽

Cited By ~ 70

Author(s):

C. KHARIF ◽

J.-P. GIOVANANGELI ◽

J. TOUBOUL ◽

L. GRARE ◽

E. PELINOVSKY

Keyword(s):

Modulational Instability ◽

Breaking Waves ◽

Short Wave ◽

Experimental Results ◽

Leeward Side ◽

Extreme Wave ◽

Time Duration ◽

Wave Event ◽

Steep Waves ◽

Extreme Wave Event

The influence of wind on extreme wave events in deep water is investigated experimentally and numerically. A series of experiments conducted in the Large Air–Sea Interactions Facility (LASIF-Marseille, France) shows that wind blowing over a short wave group due to the dispersive focusing of a longer frequency-modulated wavetrain (chirped wave packet) may increase the time duration of the extreme wave event by delaying the defocusing stage. A detailed analysis of the experimental results suggests that extreme wave events may be sustained longer by the air flow separation occurring on the leeward side of the steep crests. Furthermore it is found that the frequency downshifting observed during the formation of the extreme wave event is more important when the wind velocity is larger. These experiments have pointed out that the transfer of momentum and energy is strongly increased during extreme wave events.Two series of numerical simulations have been performed using a pressure distribution over the steep crests given by the Jeffreys sheltering theory. The first series corresponding to the dispersive focusing confirms the experimental results. The second series which corresponds to extreme wave events due to modulational instability, shows that wind sustains steep waves which then evolve into breaking waves. Furthermore, it was shown numerically that during extreme wave events the wind-driven current could play a significant role in their persistence.

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Method of the Laboratory Wave Generation for Two Dimensional Hydraulic Model Experiment in the Coastal Engineering Fields: Case of Random Waves

Journal of Korean Society of Coastal and Ocean Engineers ◽

10.9765/kscoe.2021.33.6.383 ◽

2021 ◽

Vol 33 (6) ◽

pp. 383-390

Author(s):

Jong-In Lee ◽

Il Rho Bae ◽

Young-Taek Kim

Keyword(s):

Wave Generation ◽

Hydraulic Model ◽

Coastal Engineering ◽

Experimental Results ◽

Random Wave ◽

Two Dimensional ◽

Random Waves ◽

Coastal Structures ◽

Successful Execution ◽

The Stability

The experiments in coastal engineering are very complex and a lot of components should be concerned. The experience has an important role in the successful execution. Hydraulic model experiments have been improved with the development of the wave generator and the advanced measuring apparatus. The hydraulic experiments have the advantage, that is, the stability of coastal structures and the hydraulic characteristics could be observed more intuitively rather than the numerical modelings. However, different experimental results can be drawn depending on the model scale, facilities, apparatus, and experimenters. In this study, two-dimensional hydraulic experiments were performed to suggest the guide of the test wave(random wave) generation, which is the most basic and important factor for the model test. The techniques for generating the random waves with frequency energy spectrum and the range for the incident wave height [(HS)M/(HS)T = 1~1.05] were suggested. The proposed guide for the test wave generation will contribute to enhancing the reliability of the experimental results in coastal engineering.

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