From 1971-74 seven cruises were made to measure the directional spectrum of ocean waves by using a cloverleaf buoy. Typical sets of wave data measured both in open seas and in a bay under relatively simple conditions have been analyzed to clarify the fundamental properties of the directional spectrum of ocean waves in deep water.
Analysis of the directional spectra of typical sets of surface wave data obtained in the open sea as well asa bay using a cloverleaf buoy system are reported. It is shown that the directional wave spectrum can be approximated by the product of the frequencyspectrum and a unimodal angular distribution with mean direction approximately equal to that of thewind, and that various forms of frequency spectra exist, even in relatively simple wave systems, dependingon their generating conditions. Ocean waves at fairly short dimensionless fetches show spectral forms withvery narrow spectral width, which are similar to those of laboratory wind waves. On the other hand, thespectral forms for ocean waves at very long dimensionless fetches are quite similar to the Pierson-Moskowitzspectra, which are considered, within our present data, to be the wave spectra with the largest spectral width.Finally, there exist many ocean waves at moderate dimensionless fetches, which show spectral forms with interminate spectral widths lying between the above two extremes. However, a definite relationship betweenthe spectral width and the dimensionless fetch has not been obtained in the present study. Concerning the angular distribution, it is shown that the shape of the angular distribution is dependenton the frequency of the spectral component even in a simple wave system in a generating area, althoughthe mean directions ot the spectral components are independent of the frequency and approximately equalto the wind direction. The angular distribution is very narrow for frequencies near the dominant peak of thefrequency spectrum, whereas it widens rapidly toward high and low frequencies. Thus, the major energy-containing frequency components propagate in almost the same direction as the wind with the least angularspreading. Finally, it is shown that a similarity law is satisfied for the angular distributions, and an idealized formof the angular distribution function is derived for practical purposes.
ABSTRACTMeasurements of wind waves have been done in the decay area of large scale wind flume, at one end of which a certain length was remained as the generating area. One-dimensional wave spectrum has been studied associated with the analysis of wave properties obtained directly from the wave record. The Gaussian hypothesis of wind wave was not particularly substanciated in the generating area nor in the initial part of the decay area. However, as the propagation of the waves in decay area and as the decrease of non-linearity of waves the frequency distribution approached gradually to the Gaussian distribution. Wave spectrum in decay area showed the following interesting characteristics: The dissipation of wave energy at high frequency part was very close to that due to molecular viscosity, but the dissipation of energy was much greater than that due to molecular viscosity near the dominant peak of the spectrum where some excess energy was observed as compared with the univarsal form of wave spectrum. S...
The data for the spectra of wind-generated waves measured in a laboratory tank and in a bay are analyzed using the similarity theory of Kitaigorodski, and the one-dimensional spectra of fetch-limited wind waves are determined from the data. The combined field and laboratory data cover such a wide range of dimensionless fetch F (= gF/u2 ) as F : 102 ~ 10 . The fetch relations for the growthes of spectral peak frequency u)m and of total energy E of the spectrum are derived from the proposed spectra, which are consistent with those derived directly from the measured spectra.
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