Large-scale laboratory observation of flow properties in plunging breaking waves

A plunging breaking wave of 1-m height is generated in a very large wave tank of 5 m in width, 5 m in depth, and 300 m in length. The surface velocities in the highly aerated region of the breaking wave are measured using bubble image velocimetry (BIV), while the void fraction profiles are measured using a fiber optic reflectometer (FOR). The internal velocities below the aerated region are also measured using an array of acoustic Doppler velocimeters (ADV). A wavelet-based technique is used to detect vortical structures at the free surface and estimate their length scales. The measured surface velocity fields are decomposed into wave induced and turbulence induced components to investigate the temporal and spatial evolution of mean and turbulent kinetic energy. It is found that turbulence is advected and diffused mainly following the phase speed of the breaking wave, rather than from the wave group velocity during the first splash-up process. The internal velocity measurements below the aerated regions show that turbulent kinetic energy decreases exponentially as the depth increases. To investigate the scale effects, measurements and results in the present study are compared with those from Lim et al. [2015] which analyzed small scale plunging breaking waves with a 0.2-m wave height.