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

Abstract

A plunging breaking wave of 1-m height was generated in a very large wave tank of 5 m in width, 5 m in depth, and 300 m in length filled with freshwater. The surface velocities in the highly aerated region of the breaking wave were measured using bubble image velocimetry (BIV), while the void fraction profiles were measured using fiber optic reflectometers (FOR). The internal velocities below the aerated region were also measured using an array of acoustic Doppler velocimeters (ADV). A wavelet-based technique was used to detect vortical structures at the free surface and estimate their length scales. The measured surface velocity fields were decomposed into wave induced and turbulence induced components to investigate the temporal and spatial evolution of mean kinetic energy and turbulent kinetic energy. It was 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. Since scale effects under breaking waves with turbulence and air entrainment are less understood, results in flow kinematics, turbulence, and void fraction in the present study were compared with that in Lim et al. (2015) which investigated small scale plunging breaking waves with a 0.2-m wave height. It was found that flow kinematics and some dynamic properties such as void fraction and turbulent kinetic energy can be well represented between different physical scales if the traditional Froude scaling law is applied. Other dynamic properties, including bubble number and size distributions, seem to be significantly affected by the physical scales.