A three component drag probe for the measurement of ocean wave orbital velocities and turbulent water velocity fluctuations

• Main Author: Earle, Delph Marshall, 1913-
• Other Authors: Pond, Stephen
• Language: en_US
• Published: 2012
• Subjects: Ocean currents > Measurement; Turbidity currents
• Online Access: http://hdl.handle.net/1957/28931

A three component drag probe has been built, calibrated, and used to measure velocities beneath deep water ocean waves and turbulence in a tidal channel. Simple variable inductance devices which may be submerged in water were used as displacement transducers and the associated electronics provided voltage outputs which were proportional to the three components of force that were exerted on a small 5 cm diameter sphere. The force components were due to both the water drag force and the water inertial force in an accelerating flow field. Techniques are described for interpreting measurements made with the drag probe and for obtaining the three velocity components from the measured force components. From the drag probe calibration and its use in the field, it is concluded that the drag probe is a suitable instrument for the measurement of wave velocities and turbulence. Modifications are suggested to improve the performance of the drag probe. For the wave velocity measurements, the experimental results indicate that linear wave theory is adequate to describe the relations between the wave pressure and the wave velocity components. At frequencies higher than the predominant wave frequency the velocity spectra are roughly proportional to f⁻³ where f is the frequency in Hz. The wave velocity components were used to obtain an estimate of the directional energy spectrum. From the measurements in a tidal channel, it appears that the instrument is suitable to measure turbulent fluctuations with scale sizes larger than about 20 cm. If the turbulence were isotropic the velocity spectra would be proportional to f[⁻⁵/³]. Due to the influence of boundaries, the flow was not isotropic but the results appear to be consistent with other observations that turbulent velocity spectra usually show a f⁻¹ to f⁻² behavior and are quite different from wave velocity spectra. === Graduation date: 1971