An experimental study of weakly three-dimensional non-breaking and breaking waves

Abstract

To investigate the influence of directionality on intermediate- and deep-water (kh=1.09–3.49) wave breaking, an elaborately designed physical experiment was conducted in an ‘X’ configuration, where two identical two-dimensional wave trains were generated and propagated with an approach angle of 8° (i.e. a weakly three-dimensional configuration) and encountered each other in an interaction region. The study involved measuring the wave surface elevations using calibrated wave gauges along the channels as well as recording the variation of the surface elevations spatially using video in the vicinity of the interaction region. In addition, for direct comparison a two-dimensional experiment was also conducted in one of the channels with the ‘X’ configuration essentially removed by a dummy wall in the interaction region. The experimental results show that the weakly three-dimensional experiments have little influence on the profile of wave trains with relatively low initial steepness (S0=0.215. S0=ks∑an, where ks is the wave number corresponding to the spectrally weighted wave frequency, and an is the wave amplitude of the nth wave component), although it is pronounced when breaking occurs. In addition, the frequency spectra of the focusing waves illustrate that the interaction of two wave trains increases the energy in the 0.91<f/fp<1.82 band for non-breaking cases and dissipates the energy from the higher frequency end for breaking cases. Compared with the two-dimensional experiment, it is noted that due to directional wave–wave interaction, breaking events could occur with smaller initial steepness and, more interestingly, with lower energy dissipation. It is somewhat surprising that the breaking dissipation was reduced. We noticed a shift in focusing/breaking location (i.e. premature) due to the increase in crest height using linear focusing theory, which was more pronounced during directional wave–wave interaction.