The effect of mild wind on droplet generation in a plunging breaker

An experimental study of droplet generation in a deepwater plunging breaker in the presence of initially still air (case Water) and mild wind in the wave propagating direction with two mean free stream speeds, 1.5 m/s (case W150) and 3.0 m/s (case W300), is presented. The breakers are generated by a programmable wave maker that is set with a single motion profile that produces a highly repeatable dispersively focused 2D wave packet with a central frequency of f₀ = 1.15 Hz and a corresponding wavelength of λ₀ = 1.18 m (by linear theory). The wave profiles from the time of (plunging) jet formation to the time of jet impact are measured using a high-speed laser-induced fluorescence (LIF) imaging technique. The droplets produced are measured with an in-line cinematic holographic system operating with measurement volumes that span the width of the tank. The positions, diameters (d ≥ 100 μm), times and velocities of droplets are measured as they move up across a prescribed horizontal measurement plane. The LIF measurements demonstrate strong effects of mild winds on the geometry and dynamical behaviour of the pre-breaking wave profiles. In the droplet measurement, it is found that mild winds significantly modify the overall number and the diameter and velocity distributions of the droplets associated with each of the three droplet production mechanisms, which were distinguished in a typical plunging breaker in the work of Erinin et al., J. Fluid Mech., vol.967, 2023, A36.