NUMERICAL AND EXPERIMENTAL INVESTIGATION ON THE WAVE-WAVE INTERACTION IN BREAKING AND NON-BREAKING FOCUSING WAVES

The modelling of large individual waves for the computation of loads on ships and offshore structures in extreme weather conditions is still a challenging problem. Since the early 50s the predictions of loads on fixed offshore structures and motions of compliant or sailing structures due to surface waves are commonly made by computations on the basis of the statistical/spectral description of the sea elevation and of a linearized response model. Quadratic Transfer Functions or fully non-linear methods are used only in specific cases. The linear approach is recognized to work reasonably well for the so-called operational conditions, assuming that hydrodynamic and dynamic nonlinear effects can be neglected. On the other hand, it is also recognized that the modelling of large amplitude motions and the modelling of waves in the so-called survival conditions, i.e. extreme wave conditions, cannot recast a linear approach. In these conditions the wave-wave interaction plays a fundamental role (energy transfer, down-shift, etc) in the actual deterministic or spectral representation of the wave/flow field and thus in the related loads on the structure. In the present paper the nonlinear aspects related to the behavior of steep focusing breaking and non- breaking waves are analyzed by means of numerical simulations and new experiments. The experiments are carried out at the wave flume of the Laboratory of Maritime Engineering (LABIMA) of the Dept. of Civil and Environmental Engineering of the University of Florence. The computations are carried out at Hydrodynamic and MetOcean Laboratory (HyMOLab) of the Dept. of Engineering and Architecture of the University of Trieste. The paper focuses the attention on the comparison between the results obtained with a state-of-art viscous flow simulation and laboratory experiments, with particular emphasis on the spectral energy exchange between component waves of a non-breaking and breaking focusing wave train. This study is carried out as part of the research project “OpenViewSHIP Development of an integrated computational ecosystem for the hydrodynamic design of the hull-propeller system”, co-financed by Friuli Venezia Giulia Region in the field of industrial application of open-source CFD and High Performance Computing.