Measurement of Wind Wave Related Turbulence Using an Array of Impellor and Current Meters

Abstract

A multi-faceted study has been initiated by the USCG to determine the meaningful parameters associated with an accidental oil spill. The effect of sea state in the dispersion of a slick is a major question. It is important to have quantitative estimates of the energy necessary for slick breakup and injection of oil globules downward. As one part of this investigation, the authors' contribution was to provide an observational data base for relating wind waves and turbulent mixing in the upper 5 meters. The experimental method was based on the utilization of an array of ducted impellor current meters (Smith, 1978), each 3 cm in diameter. To reduce the induced drag inherent with a conventional magnetic pickup, a Hall effect probe was used to sense the rotor motion. Specifications for new sensors list a threshold velocity of 0.5 cm/sec. Each sensor signal was averaged and then digitized at 5 Hz. This information was subsequently stored on tape cassettes located in a watertight container along with the electronics package. Two platforms were used in the collection of data: (1) a spar buoy, (2) a fixed boom. Usually six meters were placed at different depths with all axes of rotation coincident (vertical or horizontal). At times, the meters were arranged such that both vertical and horizontal velocity fluctuations were measured at particular depths. When the array was suspended from a fixed boom, the entire configuration was steered such that each velocity measurement point was at the same phase point in the observed wave field. Wave staff data were also recorded for some of the experiments. Results demonstrated an exponential decrease in kinetic energy for the upper 2 meters during nonfluctuating wind conditions. With a change in wind speed (and wave height), a corresponding change in the kinetic energy distribution with depth was noted along with an associated phase lag. Velocity spectra indicated velocity fluctuations at the wind-wave frequencies and the transfer of energy to other frequencies with increasing depth.