A Field Study of the Development of Wind-waves:Part 1. The Experiment
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This paper presents the results of observation on the development of wind-waves which were generated in a lake water about 420cm deep with a fetch 12km long. Measurements of surface elevation were carried out at the end of an observational pier where the water depth was 80cm. The wave momentum flux, i.e., the growth rate of the wave momentum, was estimated from both significant waves and power spectral densities for the wave records. The values obtained by the two ways accorded fairly well and they were 5-7% as large as the wind stress measured simultaneously. The exponential growth rate of spectral densities for a frequency component was in good accord with that observed by SNYDER and Cox (1966) and by others. If these growth rates are applied to all the components of the spectrum, the wave momentum flux must exceed the wind stress. This cannot explain the experimental results nor can be physically accepted. The difference of spectral densities between the two successive runs showed that the increase of spectral densities was limited in several bands of frequency. The phenomena are discussed in relation with the 'overshoot-undershoot effects' studied by BARNETT and SUTHERLAND (1968).<BR>Observational results suggest that the spectral growth of a certain component is closely related to the spectral densities of other components. Energy exchange among componented waves has not been considered in the theories for generation and development of wind-waves established by Phillips, Miles and others.<BR>New generation mechanism suggested by LONGUET-HIGGINS (1969) was found to be able to describe the observed growth rates of the form∅(f) ={γ(f)(t-tf)}<SUP>2</SUP>: the spectral density b (f) was proportional to the square of duration t. However, the mechanism can not explain the overshoot-undershoot effects peculiar to the equilibrium spectrum of windwaves.<BR>Three frequencies characterizing the discrete distributions of frequency bands where spectral densities increased were examined and three waves corresponding to these frequencies were found to be satisfying the resonance conditions for the 'wave-wave interactions' among three sinusoidal wave trains as studied by PHILLIPS (1960), LONGUET-HIGGINS (1962) and BENNY (1962). The interactions are suggested to predict well both the spectral growth proportional to squares of duration and the ceaseless oscillations of spectral densities in an equilibrium spectrum.
- 日本海洋学会の論文
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