Experimental Study of Kinematics of Large Transient Wave in 2-D Wave Tank

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Abstract
Increased attention has recently been focused on determination and reproduction of realistic properties of extreme sea waves for use in numerical and physical modeling and design load computations. Results of recent research on laboratory synthesis of extreme waves and experimental investigation of wave-fluid particle kinematics just prior to breaking using laser Doppler anemometry is presented. An extreme transient wave, similar to those found in hurricane Camille, and an "equivalent" regular Stokes wave often used for design purposes were generated, and their kinematics measured. Due to particular asymmetries not present in the Stokes wave, the transient wave kinematics under the crest are shown to be much more severe above the still water level and somewhat less severe below. This comparison suggests that it would be worthwhile to further investigate the use of extreme, transient waves as new, more realistic design waves. INTRODUCTION Offshore structures are diverse in their configurations, sizes and modes of moorings/fixings. Natural sea environments consisting of winds, currents and waves acting on and interacting with structures produce complex fluid loadings that are difficult to evaluate. However, practical techniques have been established for estimating wave loads under limited conditions, such as the Morison equation for slender bodies and diffraction theory for large structures. Morison's equation requires determination of inertia and drag coefficients and knowledge of the fluid kinematics resulting from wave and current interaction under the wave surface, in the absence of any structure. It is usual practice in determining wave kinematics to follow either a random wave or deterministic design-wave approach. The random wave (time series) is obtained from the energy spectra of design sea state, whereas the design wave (characterized only by height and period) is derived statistically as the most probable, largest wave at the design location for a given return period. Random wave approaches, for instance [1, 2, 3], use linear wave theory and the linear random superposition law and develop empirical formulas for determining the particle kinematics above the mean water level, using so called "stretching" techniques. The design wave can be represented by a variety of wave theories [4] including Airy, Stokes, conical, stream function, etc. Consequently, in using the foregoing wave theories, a number of particle kinematics data are generated and lead to estimates of design wave loads. Recent progress in gathering and analyzing storm sea surface data, for instance [5, 6, 7], has resulted in a significant impact on those in the research community interested in studying extreme waves and wave loadings on marine structures. For example, an analysis of data from the Gulf of Mexico hurricane "Camille" and a storm off the Irish coast [6] revealed that some of the largest waves contained in sample time histories were steep on their forward face and greatly elevated. Such waves are often termed extreme transient waves. Extreme transient waves have been reproduced in a few wave tanks, for instance [8-14, 17].

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