Characteristics of Mechanically‐Generated Waves

Abstract

The structure of a mechanically‐generated sinusoidal, water‐wave train of fixed frequency is examined under the influence of wind. The characteristics of this wave train were obtained with the aid of capacitance‐type wave height gauges in a wind‐wave research facility at Stanford University. Experimental results are given for seven wind speeds in the range 140–400 cm/s and 1 Hz, 2.54 cm (nominal) amplitude, mechanically‐generated waves. The amplitude and phase of the various wave components were deduced by a simple method using their traveling wave property and their characteristic dependence upon the streamwise position in the channel. The dispersion relation and component phase speeds were also examined. It was found that: (1) The amplitude of the forced and free‐traveling second harmonics compares favorably with existing theories; and (2) the nonlinearities of the primary wave, the interaction between short gravity waves and the primary wave, and the advection effects of wind drift are mainly responsible for the deviation of the measured phase speeds from the linear theory. The latter results are consistent with the field measurements reported by other researchers, indicating that the apparent phase speeds at high frequencies are independent of the frequency. The measured phase speeds were also found to increase with wind speed, at a given frequency, in accord with previous laboratory measurements and theoretical computations.