On the threshold for wave breaking of two-dimensional deep water wave groups in the absence and presence of wind

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 (U s ) with the wave crest velocity (C), as determined by an array of closely-spaced wave gauges. The energy flux ratio B x = U s /C that distinguishes maximum recurrence from marginal breaking was found to be 0.840±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 λ/4 /C 0 =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%.

On the influence of wave breaking on the height limits of two-dimensional wave groups propagating in uniform intermediate depth water

“…With this expanded data set, the threshold for wave breaking of intermediate water waves can be determined as 0.835 ± 0.005. Although this is slightly lower than the threshold stated by Saket et al (2017) for two-dimensional deep water wave breaking, the data of Saket et al (2017) conform to this revision. This value is also slightly lower than the value proposed numerically by Barthelemy et al (2015b).…”

“…The experiments were carried out in the two-dimensional wave tank that was 30 m long, 0.6 m wide and 0.6 m deep with glass sidewalls and a solid floor located at UNSW Sydney, Water Research Laboratory (WRL) in Manly Vale, Australia. The tank configuration was identical to that used by Saket et al (2017), except that the flexible wave paddle was replaced The thermal image velocimetry technique developed by Saket et al (2016) was used to measure the horizontal water particle velocities U s at the crests of waves transitioning through the group maximum. The entire TIV system consisted of a thermal imaging camera, an irradiating source, a computer-controlled shutter, and a computer controlling the system components.…”

“…The entire TIV system consisted of a thermal imaging camera, an irradiating source, a computer-controlled shutter, and a computer controlling the system components. Its operation is described in detail in Saket et al (2016) and its validation for monochromatic waves is presented in Saket et al (2017). Its configuration and all accompanying wave probe measurements to determine crest point speeds for this present study were identical to those used by Saket et al (2017).…”

“…Large-scale laboratory experiments have been conducted for different classes of unforced unidirectional wave groups in intermediate water depths to investigate the breaking threshold proposed by Barthelemy et al (2015b). The measurement techniques used were identical to those used by Saket et al (2017).…”

“…The water depth d was 0.35 m, 0.22 m and 0.19 m to generate intermediate water waves. Further details can be found inSaket et al (2017).…”