Laboratory study of temporal and spatial evolution of waves excited on water surface initially at rest by impulsive wind forcing

Abstract

Evolution of waves excited by wind that varies in time is not yet understood sufficiently well. In the present study, waves generated from rest by an effectively impulsive wind forcing are studied in a small laboratory wind-wave tank. Multiple parameters characterizing evolution of the wave field in time as well as in space are presented. Measurements of the variation with time of the instantaneous surface elevation were performed simultaneously with determination of two components of the instantaneous surface slope at a number of fetches along the test section. For each wind forcing conditions, numerous independent realizations were recorded. Thus, sufficient data were collected for computation of statistically reliable ensemble-averaged values of parameters characterizing the evolving random wind-wave field as a function of time elapsed since the initiation of wind. In each realization, data acquisition started when the water surface was calm, and lasted until statistically steady random wave field conditions were attained. The analysis of the ensemble-averaged wind-wave characteristics indicated that distinct stages in the wind-waves evolution could be identified. These stages were compared with the predictions based on the viscous instability theory and on the random resonant wind-waves generation model.