Predicting Ocean Rogue Waves from Point Measurements: an Experimental Study for Unidirectional Waves
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Predicting ocean rogue waves from point measurements: An experimental study for unidirectional waves The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Cousins, Will et al. "Predicting ocean rogue waves from point measurements: An experimental study for unidirectional waves." Physical Review E, 99, 3 (March 2019): 032201 © 2019 The Author(s) As Published http://dx.doi.org/10.1103/PhysRevE.99.032201 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/120744 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. PHYSICAL REVIEW E 99, 032201 (2019) Predicting ocean rogue waves from point measurements: An experimental study for unidirectional waves Will Cousins,1 Miguel Onorato,2,3 Amin Chabchoub,4 and Themistoklis P. Sapsis1,* 1Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2Dipartimento di Fisica, Università degli Studi di Torino, 10125 Torino, Italy 3Istituto Nazionale di Fisica Nucleare, INFN, Sezione di Torino, 10125 Torino, Italy 4School of Civil Engineering, The University of Sydney, Sydney NSW 2006, Australia (Received 29 April 2018; published 4 March 2019) Rogue waves are strong localizations of the wave field that can develop in different branches of physics and engineering, such as water or electromagnetic waves. Here, we experimentally quantify the prediction potentials of a comprehensive rogue-wave reduced-order precursor tool that has been recently developed to predict extreme events due to spatially localized modulation instability. The laboratory tests have been conducted in two different water wave facilities and they involve unidirectional water waves; in both cases we show that the deterministic and spontaneous emergence of extreme events is well predicted through the reported scheme. Due to the interdisciplinary character of the approach, similar studies may be motivated in other nonlinear dispersive media, such as nonlinear optics, plasma, and solids, governed by similar equations, allowing the early stage of extreme wave detection. DOI: 10.1103/PhysRevE.99.032201 I. INTRODUCTION of wave height or wave crest for the given sea state. This allows one to calculate the probability of encountering a wave Rogue waves, also known as freak waves, are abnormally whose height is larger than a certain threshold (usually 2 large waves with crest-to-trough height exceeding two times times the significant wave height); see, for example, [14]. the significant wave height [1–5]. Although rare (approxi- On shorter timescales, a deterministic prediction of rogue mately three waves per day in a single point measurement, waves is generally possible. In [15] a predictability time scale using linear theory, and an average wave period of 10 s), for rogue waves was estimated through extensive numerical these waves can have dramatic effects on ships and other simulations using a phase-resolved high-order spectral tech- ocean structures [6,7]. Therefore, predicting such extreme nique [16,17]. It was demonstrated that a timescale for reliable events is an important challenging topic in the field of ocean prediction can be O(10T ), where T is the peak period of the engineering, as well as other fields of wave physics, including p p spectrum. plasma [8], solids [9], and optics [10–12]. In addition, from a For long-crested water waves, statistics are far from Gaus- mathematical viewpoint the short-term prediction problem of sian, with heavy tails [18–21]. In this case, the dominant extreme events in nonlinear waves presents particular interest mechanism for the formation of large waves is finite-time in- due to the stochastic character of water waves and also the stabilities rising in the form of a spatially localized modulation inherent complexity of the governing equations. instability [14,22,23]. For deep water waves, a manifestation Before discussing in detail the emphasized prediction tool of this focusing is the well-known modulation instability for rogue waves, we find it relevant to make a general state- of a plane wave to small sideband perturbations [24,25]. ment on the predictability of surface gravity waves. In [13]it This instability, which has been demonstrated experimentally has been shown numerically that 2D (i.e., in two horizontal since the 1960s [26,27] and its limiting case more recently dimensions) ocean waves are described by a chaotic system; [28,29], generates significantly focused coherent structures this implies that due to positive Lyapunov exponents, after by soaking up energy from the nearby field [30–32]. In this some time (space) the system loses memory of the initial context it is possible and more advantageous to study the condition and any attempt to perform a deterministic forecast dynamics of wave groups (in contrast to individual waves) will generally fail. Annenkov and Shrira [13] found that through reduced-order representations [23,33,34], alleviating a timescale of predictability for typical steepness of ocean the direct numerical treatment of the full equations. Depend- waves is of the order of 1000 wave periods. For larger times, ing on the typical dimensions (length, width, height) of the predictions, including rogue-wave forecast, can be made only wave group, we may have subsequent modulation instability, on a statistical basis, i.e., given a wave spectrum and its which leads to further significant magnification of the wave evolution, the goal is to establish the probability distribution group height. Such critical wave groups can be formed by the random superposition of different harmonics, see Fig. 1. If a wave group has appropriate characteristics, it will amplify *Corresponding author: [email protected] due to modulation instability. Such nonlinear evolution can be 2470-0045/2019/99(3)/032201(9) 032201-1 ©2019 American Physical Society COUSINS, ONORATO, CHABCHOUB, AND SAPSIS PHYSICAL REVIEW E 99, 032201 (2019) is the modified nonlinear Schrödinger equation (MNLS) [38], formulated as an evolution equation in space rather than in time [41]. The analysis of this universal equation, that can be also applied to a wide range of physical media (for instance, in optics [42]), allows for the characterization of wave groups or pulses as critical to become either rogue or not through single point measurements of the time series of the surface elevation. We demonstrate the effectiveness of the developed scheme through experimental hydrodynamic data in the form of time series of water wave profiles. Using multiple realizations of rogue waves, we statistically quantify the accuracy of the developed scheme. The main distinction of the present work is that in prior work the predictive scheme was applied only in the context of forecasting the propagation of a wave field through numerical FIG. 1. In the typical regime the dominant mechanism for the simulations of the MNLS. It is true that the laboratory exper- wave group formation is the superposition of linear waves. If a iments considered here are overly simplistic representations critical wave group is formed, i.e., one with sufficiently large length of realistic ocean dynamics. However, the work presented and amplitude, the strongly nonlinear dynamics associated with here represents a significant step forward in reduced-order modulation instability can be foreseen through simple precursors. forecasting of extreme events, demonstrating that this scheme can provide accurate spatiotemporal predictions in an experi- foreseen using simple precursors that quantify the conditions mental environment with noisy measurements. for modulation instability of the wave group, as shown in Fig. 1. II. PRECURSORS BASED ON POINT MEASUREMENTS A reduced-order precursor for the prediction of rogue waves, caused by spatially localized modulation instability, Our goal is to predict extreme waves in unidirectional wave has been proposed for unidirectional [35,36] as well as direc- fields on the surface of deep water using time measurements tional [37] surface gravity waves. The idea behind it comes at a single point with satisfactory high sampling frequency. from combining spectral information for the sea state and The developed scheme consists of an offline, as well as an information involving the evolution of isolated wave groups online, real-time component. For the offline component, we to rogue waves. The derived precursors have the form of quantify the critical wave groups that evolve to rogue waves using direct numerical solutions of the MNLS equation. Here characteristic patterns that precede rogue waves O(10Tp) ahead. Using field information (i.e., wave measurements with we employed the MNLS equation for demonstration purposes; spatial extend) for the region of interest, the predictive scheme the fully nonlinear water wave equations could also be used, quickly identifies locations where these patterns are present but the offline component would be computationally more and provides the estimated magnitude of a rogue wave that expensive. In the online, real-time component, we identify the will occur in the near future, close to this location. The coherent wave groups in measurements of a physical, irregular developed scheme is particularly robust, given that it relies wave time-series. We then use the results from the offline on the detection of large-scale features (having the size of component to predict how the measured groups will evolve. the wave group) utilizing either temporal or spatial mea- The scheme we discuss here closely follows the ideas surements. For this reason the scheme does not depend on presented in [35]. In this case the prediction analysis was small-scale measurement errors. In addition, it is extremely based on the availability of field measurements. The algorithm fast due to the fact that there is no need to calculate any reported in this work predicts future extreme waves from solution of any evolution equation involved in the prediction time-series measurements of the wave field at a single point.