Alexey Slunyaev

Nonlinear wave focusing on water of finite depth

Abstract The problem of freak wave formation on water of finite depth is discussed. Dispersive focusing in a nonlinear medium is suggested as a possible mechanism of giant wave generation. This effect is considered within the framework of the nonlinear Schrodinger equation and the Davey–Stewartson system, describing 2+1-dimensional surface wave groups on water of finite depth. In the 2+1-dimensional case, the dispersive grouping is accompanied with a geometrical focusing. Necessary wave conditions for the occurrence of such a phenomenon are discussed. Influence of non-optimal phase modulation and presence of strong random wave component are found to be weak: they do not cancel the mechanism of wave amplification. The mechanism of dispersive focusing is compared with the wave enhancement due to the Benjamin–Feir instability, which is found to be extremely sensitive with respect to weak random perturbations.

Focusing of nonlinear wave groups in deep water

The freak wave phenomenon in the ocean is explained by the nonlinear dynamics of phase-modulated wave trains. It is shown that the preliminary quadratic phase modulation of wave packets leads to a significant amplification of the usual modulation (Benjamin-Feir) instability. Physically, the phase modulation of water waves may be due to a variable wind in storm areas. The well-known breather solutions of the cubic Schrödinger equation appear on the final stage of the nonlinear dynamics of wave packets when the phase modulation becomes more uniform.

Applicability of envelope model equations for simulation of narrow-spectrum unidirectional random wave field evolution: Experimental validation

Combined experimental and numerical study of spatial evolution of unidirectional random water-waves is performed. Numerous realizations of wave fields all having identical initial narrow-banded Gaussian power spectrum but random phases for each harmonic were generated by a wavemaker in a 300 m long wave tank. The measured in the vicinity of the wavemaker temporal variation of the surface elevation was used to determine the initial conditions in the numerical simulations. The cubic Schrodinger equation (CSE) and the modified nonlinear Schrodinger (MNLS) set of equations due to Dysthe were used as the theoretical models. The detailed comparison of the evolution of the wave field along the tank in individual realizations, measured by wave gauges at different distances from the wavemaker and computed using the two theoretical models, was performed. Numerous statistical wave parameters were calculated based on the whole ensemble of realizations. Comparison of the spatial variation of the computed statistical c...

Super-rogue waves in simulations based on weakly nonlinear and fully nonlinear hydrodynamic equations

The rogue wave solutions (rational multibreathers) of the nonlinear Schrödinger equation (NLS) are tested in numerical simulations of weakly nonlinear and fully nonlinear hydrodynamic equations. Only the lowest order solutions from 1 to 5 are considered. A higher accuracy of wave propagation in space is reached using the modified NLS equation, also known as the Dysthe equation. This numerical modeling allowed us to directly compare simulations with recent results of laboratory measurements in Chabchoub et al. [Phys. Rev. E 86, 056601 (2012)]. In order to achieve even higher physical accuracy, we employed fully nonlinear simulations of potential Euler equations. These simulations provided us with basic characteristics of long time evolution of rational solutions of the NLS equation in the case of near-breaking conditions. The analytic NLS solutions are found to describe the actual wave dynamics of steep waves reasonably well.

Generation of solitons and breathers in the extended Korteweg-de Vries equation with positive cubic nonlinearity

The initial-value problem for box-like initial disturbances is studied within the framework of an extended Korteweg-de Vries equation with both quadratic and cubic nonlinear terms, also known as the Gardner equation, for the case when the cubic nonlinear coefficient has the same sign as the linear dispersion coefficient. The discrete spectrum of the associated scattering problem is found, which is used to describe the asymptotic solution of the initial-value problem. It is found that while initial disturbances of the same sign as the quadratic nonlinear coefficient result in generation of only solitons, the case of the opposite polarity of the initial disturbance has a variety of possible outcomes. In this case solitons of different polarities as well as breathers may occur. The bifurcation point when two eigenvalues corresponding to solitons merge to the eigenvalues associated with breathers is considered in more detail. Direct numerical simulations show that breathers and soliton pairs of different polarities can appear from a simple box-like initial disturbance.

Stochastic simulation of unidirectional intense waves in deep water applied to rogue waves

The results of numerical and laboratory simulation of ensembles of quasi-random unidirectional intense surface gravity waves in deep water have been summarized. The role of nonlinear self-modulation of the waves applied to the problem of ocean rogue waves, as well as the appearance, dynamics, and manifestation of non-linear wave packets in stochastic wave fields, is discussed.

Wave amplification in the framework of forced nonlinear Schrödinger equation: The rogue wave context

Abstract Irregular waves which experience the time-limited external forcing within the framework of the nonlinear Schrodinger (NLS) equation are studied numerically. It is shown that the adiabatically slow pumping (the time scale of forcing is much longer than the nonlinear time scale) results in selective enhancement of the solitary part of the wave ensemble. The slow forcing provides eventually wider wavenumber spectra, larger values of kurtosis and higher probability of large waves. In the opposite case of rapid forcing the nonlinear waves readjust passing through the stage of fast surges of statistical characteristics. Single forced envelope solitons are considered with the purpose to better identify the role of coherent wave groups. An approximate description on the basis of solutions of the integrable NLS equation is provided. Applicability of the Benjamin–Feir Index to forecasting of conditions favourable for rogue waves is discussed.

Laboratory and numerical study of intense envelope solitons of water waves: Generation, reflection from a wall, and collisions

The investigation of dynamics of intense solitary wave groups of collinear surface waves is performed by means of numerical simulations of the Euler equations and laboratory experiments. The processes of solitary wave generation, reflection from a wall, and collisions are considered. Steep solitary wave groups with characteristic steepness up to kAcr ≈ 0.3 (where k is the dominant wavenumber and Acr is the crest amplitude) are concerned. They approximately restore the structure after the interactions. In the course of the interaction with the wall and collisions, the maximum amplitude of the wave crests is shown to enhance up to 2.5 times. A standing-wave-like structure occurs in the vicinity of the wall, with certain locations of nodes and antinodes regardless the particular phase of the reflecting wave group. A strong asymmetry of the maximal wave groups due to an anomalous setup is shown in situations of collisions of solitons with different frequencies of the carrier. In some situations of head-on col...

Standing Gravity Wave Regimes in a Shallow-Water Resonator

Arising modulations of surface gravity waves in a shallow-water resonator under harmonic forcing is discovered in laboratory experiments. Different types of modulations are found. When certain conditions are satisfied (appropriate frequency and sufficient force of excitation), the standing waves become modulated, and the envelopes of standing waves propagate in the channel. Strongly nonlinear numerical simulations of the Euler equations are performed reproducing the modulational regimes observed in the laboratory experiments. The physical mechanism responsible for the occurrence of modulated waves is determined on the basis of the simulations; quantitative estimates are made with the help of a simplified weakly nonlinear theory. This work was initiated by and performed under the guidance of Prof. A. Ezersky. We dedicate this text to the memory of him.

The pressure field beneath intense surface water wave groups

Abstract A weakly-nonlinear potential theory is developed for the description of deep penetrating pressure fields caused by single and colliding wave groups of collinear waves due to the second-order nonlinear interactions. The result is applied to the representative case of groups with the sech-shape of envelope solitons in deep water. When solitary groups experience a head-on collision, the induced due to nonlinearity dynamic pressure may have magnitude comparable with the magnitude of the linear solution. It attenuates with depth with characteristic length of the group, which may greatly exceed the individual wave length. In general the picture of the dynamic pressure beneath intense wave groups looks complicated. The qualitative difference in the structure of the induced pressure field for unidirectional and opposite wave trains is emphasized.