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Observations of normal pressure on windgenerated sea waves Dobson, Frederick William

Abstract

Supervisor: Professor Robert W. Stewart The process by which the wind makes sea waves grow is not well-understood, partly because of the lack of adequate observational information on the normal pressures which transfer energy to the waves. The principal object of this experiment has been to provide some of the missing data. A system for making simultaneous measurements of normal pressure and wave height was developed and tested in the laboratory and in the field. The system consisted of a disc-shaped buoy 23 cm in diameter (in which was embedded the pressure sensor) which rode up and down on a vertical rod, which was the wave sensor. Careful attention was paid to rejecting so-called "dynamic" pressures associated with the distortion of the air flow by the buoy. The results from the experiment are presented as power and cross-spectra of the pressure and wave signals. Spectra of Energy (E) and Momentum (τω) fluxes to the waves, and of ζ, the fractional energy increase of the waves per radian, are also presented. Wave power spectra are found to be normal for the site; the pressure power spectra consist of a "basic" spectrum similar to that observed over land, on which is superimposed a wave-induced "hump". The phase angle between the waves and the pressure at the frequency of the peak of the wave spectrum is found to be shifted from -180° (pressures high over wave troughs) by amounts which exceed the theoretical predictions of Miles (1957) by an average of 20 ± 5° over a wide range of conditions. The Ė and τω spectra are found to be sharply peaked at or above the frequency of the peak of the wave spectrum. The integrated energy fluxes Ē show large scatter, indicating that the wave generation process varies considerably in time (and space). The integrated momentum fluxes τω to the waves show no significant difference from total fluxes from air to water computed assuming a constant drag coefficient of 1.2 x 10⁻³; it appears that about 80% of the total drag of the water on the wind is caused by the wave generation process. The ζ spectra exceed the predictions of Miles' (1957) theory by factors of 5 to 8, indicating that his "inviscid laminar" model is not adequate to explain observed rates of wave growth. The present results fall close to an empirical curve suggested by Snyder and Cox (1966) except at high frequencies, where they are considerably lower. A dimensionless plot of ζ versus the ratio of wind speed to wave speed is presented; the observed data is fitted by the simple relation [formula omitted], where [formula omitted]is the ratio of the densities of air and water, U₂ is the mean wind speed at a height of two meters, and c is the phase velocity of the waves. This formula is only considered applicable for U₅/c < 6, where U₅ is the mean wind speed at 5 meters height. Also presented are the results of a dry-land comparison of the buoy pressure sensor with two other pressure sensors; besides indicating that the buoy sensor was adequate, this comparison produced some interesting preliminary information on the vertical and horizontal structure of the turbulent pressure field in the atmospheric boundary layer.

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