An Autonomous Ocean Turbulence Measurement Platform

Abstract

The use of a small autonomous underwater vehicle (AUV) as a platform for making in situ flow measurements in the ocean environment is described. Two high-wavenumber shear probes and a dynamic Pitot tube, housed in a pressure vessel mounted on the nose of the AUV, allow measurements, in the dissipation range, of all three components of velocity. Microstructure temperature and possible body vibrations are monitored using auxiliary local probes. A conductivity?temperature?depth package, an acoustic Doppler current profiler, and a Marsh?McBirney current meter on board allow measurement of mean background conditions. Gathered data are stored on an onboard computer. The AUV can survey 7?11-km regions at a speed of 1.5?2 m s?1, its motion being uncoupled from that of any surface mother ship. The vehicle has relatively low manufacture and operational costs and can potentially operate in stormy conditions. The small-scale measurements allow the determination of estimates of in situ energy dissipation rates. Turbulence measurements in shallow water off the east coast of Florida are described. Analysis shows that the gathered data are of high quality, the measured cross-stream shear spectra being in very good agreement with the Nasmyth spectrum. The agreement suggests that the custom-designed, self-noise isolation system that has been implemented is well engineered and has rendered the AUV platform?s self-noise acceptably low for making turbulence measurements. Use of a small AUV for flow measurement promises to be an inexpensive, practical way of making a four-dimensional survey of significant regions of the ocean.