Development of an acoustic vorticity meter to measure shear in ocean-boundary layers

Abstract

This thesis describes the analysis and development of an acoustic vorticity meter to measure shear in ocean-boundary layers over smaller measurement volumes than previously possible. A nonintrusive measurement of vorticity would filter out irrotational motion such as surface waves and currents that can swamp small scale measurements of shear. The thesis describes the desired geophysical measurements and translates this oceanographic context into design goals. The instrument was designed, built, tested, and deployed. It measures three-axis vorticity at 0.83 and 2.45 meters below the ocean surface with measurement volumes of 0.45 meters on a side. The instrument forms a buoy that is inertially instrumented to calculate and remove buoy motion from the measurements. The instrument uses a complementary filter algorithm to estimate attitude and motion from low-power, inexpensive, strapdown rate gyros, accelerometers, and fluxgate magnetometers. The instrument performance has been measured to have a vorticity bias of not more than 1 x 10-2 per second in a mean flow of 0.7 meters per second, a bias of not more than 1 x 10-2 per second in the down-wave and vertical directions in typical ocean waves, and a 30 decibel spectral rejection of surface wave velocity. Two instrument deployments are described to show the potential of the system. The instrument has measured shear in the upper-ocean-boundary layer, and these measurements are compared to concurrently measured wind stress and stratification. The instrument was also deployed, tethered in the thermocline, in an area of high internal wave activity. Richardson-number time series were measured and compared favorably to concurrently measured Richardson numbers made over a larger spatial scale.