Water waves excited by near-impulsive wind forcing

Abstract

Only limited information is currently available on the evolution of waves generated by wind that varies in time, and in particular on the initial stages of wind–wave growth from rest under a suddenly applied wind forcing. The emerging wind–wave field varies in time as well as in space. Detailed knowledge of wave parameter distributions under those conditions contributes to a better understanding of the mechanisms of wind wave generation. In the present study, the instantaneous surface elevation and two components of the instantaneous surface slope were recorded at various fetches in a small-scale experimental facility under nearly impulsive wind forcing. Numerous independent realizations have been recorded for each selection of operational conditions. Sufficient data at a number of fetches were accumulated to calculate reliable ensemble-averaged statistical parameters of the evolving random wind–wave field as a function of the time elapsed from activation of wind forcing. Distinct stages in the wave evolution process from appearance of initial ripples to emergence of a quasi-steady wind–wave field were identified. The experimental results during each stage of evolution were analysed in view of the viscous instability theory by Kawai (J. Fluid Mech., vol. 93, 1979, pp. 661–703) and the resonance model by Phillips (J. Fluid Mech., vol. 2, 1957, pp. 417–445).