Dissipation regimes for short wind waves

Abstract

[1] The dissipation processes affecting short wind waves of centimeter and decimeter scales are investigated experimentally in laboratory. The processes include damping due to molecular viscosity, generation of capillary waves, microbreaking, and breaking. The observations were made in a large wind wave tank for a wide range of fetches and winds, using a laser sheet and a high-resolution video camera. The work aims at constructing a comprehensive picture of dissipative processes in the short wind wave field, to find for which scales particular dissipative mechanism may become important. Four distinct regimes have been identified. For capillary-gravity wave fields, i.e., for dominant waves with scales below 4 cm, viscous damping is found to be the main dissipation mechanism. The gravity-capillary wave fields with dominant wavelength less than 10 cm usually exhibit a train of capillary ripples at the crest wavefront, but no wave breaking. For such waves, the main dissipation process is molecular viscosity occurring through nonlinear energy cascade toward highfrequency motions. Microscale breaking takes place for waves longer than 10 cm and manifests itself in a very localized surface disruption on the forward face of the crest. Such events generate turbulent motions in water and thus enhance wave dissipation. Plunging breaking, characterized by formation of a crest bulge, a microjet hitting the water surface and a splash-up, occurs for short gravity waves of wavelength exceeding 20 cm. Macroscale spilling breaking is also observed for longer waves at high winds. In both cases, the direct momentum transfer from breaking waves to the water flow contributes significantly to wave damping.

Capillary effects on wave breaking

“…In the case of mechanically generated parasitic-capillary waves, both the wavelength and amplitude of the carrier wave are initial parameters of the experiment. Finally, for the wind generated waves, both the wavelength and the amplitude can be extracted from the figures of Zhang (1995) and Caulliez (2013). Figure 11 shows the boundaries of the wave regime diagram of figure 7, together with the experimental data summarized in table 1.…”

“…This tends to confirm the idea that spilling breakers at large Bond number (large scale) is a transition regime between non-breaking and plunging wave. On the other hand, the wind generated small scale spilling breakers described by Caulliez (2013) correspond to smaller Bond number (Bo < 100) and larger slope and are located inside the spilling zone (S-B).…”

“…They provide qualitative and quantitative comparisons with the various wave patterns and phenomenology observed experimentally. The enhancement of dissipation by parasitic capillaries has also been described, both experimentally (Zhang 1995;Caulliez 2013) and numerically Tsai & Hung 2007Melville & Fedorov 2014). Numerical simulations of the complete two-phase flow remain relatively rare and have focused on the resolution of the wave breaking and post impact dynamics, which are numerically challenging (Chen et al 1999;Song & Sirviente 2004;Iafrati 2009Iafrati , 2011Iafrati et al 2013).…”

“…The understanding of dissipation induced by breaking waves at various scales has been the topic of intensive research in recent decades, combining field work in the open ocean (Melville 1996;Melville & Matusov 2002;Veron et al 2008;Sutherland & Melville 2013), laboratory experiments on wave breaking (Melville & Rapp 1985;Tulin & Waseda 1999;Rapp & Melville 1990;Banner & Peirson 2007;Drazen et al 2008), and the dissipation induced by smaller scales involving surface tension, like micro-breaking, spilling breaking (Duncan et al 1999;Duncan 2001;Liu & Duncan 2003, 2006 and parasitic capillary waves (Perlin et al 1993;Jiang et al 1999;Caulliez et al 1998;Su 1982;Caulliez 2013). The importance of small scales, O(1 − 10) cm in the global ocean wave energy fluxes is still an open question and needs further investigation.…”

Ka‐band backscattering from water surface at small incidence: A wind‐wave tank study

“…At very small wind speeds, D(f) values observed just above 10 Hz are very similar to the dominant wave ones, except above 40–50 Hz where this ratio starts to increase continuously. This reflects the fact that the parasitic bound waves generated at the crest of dominant waves by a nonlinear instability mechanism as described theoretically by, e.g., Tsai and Hung [] have fundamentally the same three‐dimensional features as the carrier waves and propagate at the same phase speed [ Caulliez , ]. In that respect, the D(f) increase observed above 50 Hz may result from the fact that the ripple harmonics in the crosswind direction are more rapidly damped than these in the wind direction, the energy of the latter being directly supplied by wind.…”