SURFACE RIPPLES DUE TO STEADY BREAKING WAVES

Breaking waves generated by a two-dimensional hydrofoil moving near a free surface at constant speed (U-infinity), angle of attack and depth of submergence were studied experimentally. The measurements included the mean and fluctuating shape of the breaking wave, the surface ripp les downstream of the breaker and the vertical distribution of vertica l and horizontal velocity fluctuations at a single station behind the breaking waves. The spectrum of the ripples is highly peaked and shows little variation in both its peak frequency and its shape over the fi rst three wavelengths of the wavetrain following the breaker. For a gi ven speed, as the breaker strength is increased, the high-frequency en ds of the spectra are nearly identical but the spectral peaks move to lower frequencies. A numerical instability model, in conjunction with the experimental data, shows that the ripples are generated by the she ar flow developed at the breaking region. The spectrum of the vertical velocity fluctuations was also found to be highly peaked with the sam e peak frequency as the ripples, while the corresponding spectrum of t he horizontal velocity fluctuations was found not to be highly peaked. The root-mean-square (r.m.s.) amplitude of the ripples (eta(rms)) inc reases with increasing speed and with decreasing depth of submergence of the hydrofoil, and decreases as x(-1/2) with increasing distance x behind the breaker. The quantity (g eta(rms))/(UinfinityVrms) (where V -rms is the maximum r.m.s. vertical velocity fluctuation and g is the gravitational acceleration) was found to be nearly constant for all of the measurements.