Wave breaking onset of two-dimensional deep-water wave groups in the presence and absence of wind

TL;DR

This study experimentally investigates the wave breaking onset criterion in deep-water wave groups, examining effects of wind and spectral bandwidth, and finds a consistent energy flux ratio threshold with minor variations due to wind and bandwidth changes.

Contribution

It extends the numerical wave breaking criterion to laboratory experiments, validating the energy flux ratio threshold under various wind and spectral conditions.

Findings

The energy flux ratio threshold for breaking is approximately 0.84.

Wind forcing increases the threshold by about 2%.

Reducing spectral bandwidth decreases the threshold by 1.5%.

Abstract

The criterion for the initiation of breaking demonstrated numerically by Barthelemy et al. (2015) has been investigated in the laboratory for unidirectional wave groups in deep-water and extended to include conditions of moderate wind forcing. Thermal Image Velocimetry was used to compare measurements of the crest surface water particle velocity (Us) with the wave crest velocity (C), as determined by an array of closely-spaced wave gauges. The energy flux ratio Bx = Us/C that distinguishes maximum recurrence from marginal breaking was found to be 0.840 $\pm$ 0.016 in good agreement with the numerically determined value of 0.855. Further, the threshold was found to be robust for different classes of wave groups of distinct characteristic steepness at the breaking threshold. Increasing wind forcing from zero to U{\lambda}/4/C0=1.42 increased this threshold by 2%. Increasing the spectral bandwidth (decreasing the Benjamin-Feir index from 0.39 to 0.31) systematically reduced the threshold by 1.5%.