Laboratory studies of nonlinear and breaking surface waves

A laboratory investigation of nonlinear and breaking surface waves is presented in two parts. The first focuses on the instability of progressive surface gravity waves incident on a vertical wall and the second on the measurement of the kinematics and dynamics of breaking progressive waves and the turbulence they generate.;In Part I, theoretical arguments suggest that progressive gravity waves incident on a vertical wall can produce periodic standing waves only if the incident wave steepness ak is quite small. Laboratory experiments are carried out in which an incident wave train of almost uniform amplitude meets a vertical barrier. When ak > 0.236, a growing instability is observed in which every third wave crest is steeper than its neighbours. The instability grows by a factor of about 2.2 for every three wave periods, almost independently of the incident wave steepness.;In Part II, the measurement of the dissipation of wave energy by breaking over a significant range of parameter space allows the kinematics of breaking to be related to the underlying dynamics. Control volume analysis yields a measure of the change in energy flux across the volume and is related to the dissipation through the duration of active breaking. Assuming the plunging wave toe follows a ballistic trajectory, an inertial estimate of the dissipation is developed and found to predict the dissipation rate within an order of magnitude.;Detailed measurements of the post-breaking velocity field using DPIV are conducted in the longitudinal and transverse planes. Statistical measures of the turbulence are presented. Separation of the surface-wave induced velocity from the full measured velocity helps isolate the effects of breaking, including the generation of coherent vorticity. Turbulent wavenumber spectra exhibit a deviation from the inertial subrange at high wavenumbers, thought to be caused by an imbalance between the flux of energy from large scales and the dissipation at small scales. Measurements of terms in the turbulent kinetic energy density equation are presented. The relationship between the three-dimensional turbulent kinetic energy density and two-dimensional approximations are discussed. A comparison between various estimates of the rate of viscous dissipation is also given.