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

The nonlinear effects deriving from the focussing of unidirectional waves p ropagating 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 s hown that in the frame of the inviscid fluid hypothesis, the nonlinear inte raction of waves leads to an evident space variation of the amplitude spect rum with a large magnification factor of the focused wave elevation compare d to the result from linear superposition, a phase shift of the component w aves occurs and part of the energy content is shifted to frequencies well a bove the highest frequency in the input spectrum. These features have basic consequences on the safety and design aspects of ships and marine structur es: for instance, this regards the ringing response of large offshore struc tures. At the same time the availability of kinematic data from the simulat ion of large waves can be useful for the evaluation of the forces exerted o n slender surface piercing bodies in inertia-drag regime method without sem i-empirical stretching methods. Moreover the controlled generation of deter ministic wave groups with a given frequency content, aimed at the developme nt of new techniques for seakeeping tests, is one of the subjects of intere st in ITTC (23rd International Towing Tank Conference). (C) 2001 Elsevier S cience Ltd. All rights reserved.