Simultaneous measurements of the sea surface displacement and the longitudinal component of the wind velocity at several levels are reported. They were obtained at the Marine Tower under various conditions, with the air and the waves moving either in the same or in opposite directions. The spectral analysis was made. The cross-correlation coefficient between the sea surface displacement and the wind velocity is large at the layer adjacent to the surface and decreases with increasing mean wind velocity and height. Below a certain level which is several times or several tens of times higher than the height of the critical level where the wind velocity component in the direction of wave propagation equals the wave velocity, the phase lag of the Fourier component of the wind velocity compared with the surface elevation component is about 160 to 190°. Above this layer the wave-induced wind component is very weak and the phase reversal takes place at the height where the mean wind velocity equals 1·2 to 1·5C, C being the phase velocity of wave. When the wind blows in the opposite direction from that of the wave propagation, the wind fluctuation is in phase with respect to the wave motion and the amplitude of wave-induced wind component is relatively large. Some discrepancies are shown between the observations and the predictions from the theory of inviscid fluids.

Taking account of the force arising from the relative wind velocity between the cup and the wind, the differential equation of rotation of cup assembly was formulated. Dynamic responses of cup anemometers to the stepwise and sinusoidal wind changes and to turbulent fluctuations of wind speed were numerically examined. And the damping rate of amplitude and the phase lag in measuring fluctuating wind, and the over-estimation factor in measuring mean wind speed were estimated. These characteristics of the cup anemometer may be expressed according to two constants, which are the calibration constant and the response distance of anemometer.

Cross correlations of horizontal wind fluctuations between points separated in space were measured at the Marine Observation Tower with a network of three-dimensionally distributed cup-anemometers.The phase shift of wind fluctuations between vertically separated points, and the coherence of wind fluctuations between points arranged in vertical, lateral and longitudinal directions were calculated. The vertical, lateral and longitudinal sizes of the turbulent eddy were empirically formulated in terms of the vertical, lateral and longitudinal coherences, respectively. A model of three-dimensional structure of horizontal wind components is represented by a distorted cylindrical form.Taylor's hypothesis of frozen turbulence is confirmed to hold for every frequency (0.01 Hz

No abstract

No abstract