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.

On Evolution of Young Wind Waves in Time and Space

“…The particles' positions were collected from each frame, and consequently, two velocity components-along the tank, U s and across the tank, V s -were tracked across all frames of appearance. Numerous measurements were performed to obtain statistically reliable mean values for different operational conditions [51,67].…”

“…All experiments were controlled by a LabView program that enables running the experiment, including setting the wind speed in the tunnel, the vertical positioning of the air sensors, the calibration of the sensors before and after data acquisition sessions, and the data acquisition, in a fully automatic mode without human intervention. More information about the experimental facility, calibration of the sensors, and of the detailed experimental and data-processing procedures is provided in [51,63,66,67].…”

“…However, only limited experimental data on waves under time-dependent wind under controlled conditions are currently available; these works were briefly reviewed in the Introduction. This present section is based on the experiments carried out by Zavadsky and Shemer [67]; it presents time-resolved statistically reliable results on waves excited from rest by a (nearly) impulsive wind forcing. These findings were further discussed in [97].…”

“…The scale corresponding to the maximum intensity of the ensemble-averaged map defines the dominant pseudo-frequency f dom at each instant. More details on the determination of f dom (t) are given in [67].…”

“…The slope oscillations in the crosswind direction lag by less than 1 s, while the surface elevation oscillations develop much slower. The delay in the emergence of crosswind undulations can plausibly be associated with the Langmuir circulation excited in water by accelerated wind [45,98,99], although the attempts to detect this circulation in [67] were indecisive. Quasi-steady levels of slope fluctuations in both directions are of comparable values and were attained at similar times, which were notably earlier than that of η.…”