Transition of mechanically generated regular waves to wind waves under the action of wind

Abstract

The transition process of mechanically generated regular waves into wind waves is investigated in a wind-wave tunnel. Modulations of characteristic quantities of the waves are examined in both space and time using wave-gauge arrays. The transition begins with the occurrence of wave breaking, and it is associated with the following two processes: (i) the irregularization, i.e. the generation and amplification of a random modulation whose wave height and period are in phase; and (ii) the frequency shift to the lower side by the mutual coalescence of waves through the amplification of the modulation, the coalescence occurring at the troughs of the period modulation. The modulational properties under the effect of wind with wave breaking are thus in part different from the theoretical prediction of modulational instability of a freely travelling wavetrain. It is concluded that the mechanism of the transition is the modulational instability coupled with the wind effects.