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Direct measurements of stress and spectra of turbulence in the boundary layer over the sea Weiler, Henry Sven

Abstract

The work carried out for this thesis forms part of the air-sea interaction program, which has been under way since 196l at the Institute of Oceanography of the University of British Columbia. Measurements of fluctuations in the vertical and horizontal components of air velocity were made using hot wires in an X-array, in order to study the spectra of the fluctuations, and their co-spectrum over a range of mean wind speeds from 140 – 1000 cm./sec. in the boundary layer over the sea. In order to use the X-wire probe properly in the field, special techniques were developed to mount and calibrate the wires, and to measure directly their responses to the two velocity fluctuations. Analog techniques were developed to analyze the hot wire signals, and final calculations were made by digital computer. Single (U-wire) hot wire probes were used to measure the horizontal velocity fluctuations to check the behaviour of X-wires, and to provide additional checks on the similarity theory of turbulence. Measurements showed that X-wire techniques can be used successfully to measure velocity fluctuations in two directions in the field. Hot wires have responses which give spectral levels which are accurate only within about 35%, but comparison of the horizontal velocity spectrum measured simultaneously with the X- and U-wire probes showed that their spectral shapes were similar, giving confidence in the X-wire measurements. In the high frequency range, the observed spectra of the two velocity fluctuations did not conform to the theoretical predictions. The observed behaviour is believed to be real. The cospectrum gives a direct estimate of contributions to the Reynolds' stress by fluctuations in small ranges of frequency. The stress observed between the frequency limits O.Ol6 to 10 Hz had significant contributions over about one frequency decade, which apparently lies entirely within these extremes. Estimates of the frequencies of dominant waves at the experimental site fell between about 0.2 to 0.5 Hz. Significant stress was present in this interval, but the largest proportion of the observed stress was present at lower frequencies. Ten direct estimates of stress were obtained with the X-wire. Values estimated indirectly from the wind profiles tended to give low estimates and were poorly correlated with the direct estimates. Values determined indirectly using the inertial subrange appeared to be consistently related to the directly estimated stress, but overestimated it by about 40%. Drag coefficients corrected to the 5m height were near 1.5 x 10⁻³ for wind speeds between 1.4 and 10m.sec-¹. Measurements by three U-wires spaced vertically, provided confirmation of the validity of the Monin-Obukhov similarity theory at heights below about 5m.

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