Nonlinear effects in 2D transient nonbreaking waves in a closed flume

The nonlinear effects deriving from the focussing of unidirectional waves propagating in a wave flume are analysed by direct fully nonlinear numerical simulation. The main aim of this study is to evidence the features of the wave-wave interaction in the spatial region, where the maximum elevation of the component waves of a given wave group is almost in phase. It will be shown that in the frame of the inviscid fluid hypothesis, the nonlinear interaction of waves leads to an evident space variation of the amplitude spectrum with a large magnification factor of the focused wave elevation compared to the result from linear superposition, a phase shift of the component waves occurs and part of the energy content is shifted to frequencies well above the highest frequency in the input spectrum. These features have basic consequences on the safety and design aspects of ships and marine structures: for instance, this regards the ringing response of large offshore structures. At the same time the availability of kinematic data from the simulation of large waves can be useful for the evaluation of the forces exerted on slender surface piercing bodies in inertia-drag regime method without semi-empirical stretching methods. Moreover the controlled generation of deterministic wave groups with a given frequency content, aimed at the development of new techniques for seakeeping tests, is one of the subjects of interest in ITTC (23rd International Towing Tank Conference). (C) 2001 Elsevier Science Ltd. All rights reserved.