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Measurement of Turbulence in the Oceanic Bottom Boundary layer with an Acoustic Current Meter Array

Albert J. Williams 3rdDepartment of Ocean Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

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John S. TochkoDepartment of Ocean Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

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Richard L. KoehlerDepartment of Ocean Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

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William D. GrantDepartment of Ocean Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

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Thomas F. GrossDepartment of Ocean Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

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Christopher V. R. DunnDepartment of Ocean Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

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Abstract

A vertical array of acoustic current meters measures the vector flow field in the lowest 5 m of the oceanic boundary layer. By resolving the velocity to 0.03 cm s−1 over 15 cm paths, it samples the dominant turbulent eddies responsible for Reynolds stress to within 50 cm of the bottom. Profiles through the inner boundary layer, from six sensor pods, of velocity, turbulent kinetic energy, and Reynolds stress can be recorded for up 10 four months with a 2 Hz sample rate and 20 min averaging interval. We can study flow structure and spectra from as many as four event-triggered recordings of unaveraged samples, each lasting one hour, during periods of intense sediment transport. Acoustic transducer multiplexing permits 24 axes to be interfaced to a single receiving circuit. Electrical reversal of transducers in each axis eliminates zero drift. A deep-sea tripod supports the sensor array rigidly with minimum flow disturbance, yet releases on command for free vehicle recovery.

Abstract

A vertical array of acoustic current meters measures the vector flow field in the lowest 5 m of the oceanic boundary layer. By resolving the velocity to 0.03 cm s−1 over 15 cm paths, it samples the dominant turbulent eddies responsible for Reynolds stress to within 50 cm of the bottom. Profiles through the inner boundary layer, from six sensor pods, of velocity, turbulent kinetic energy, and Reynolds stress can be recorded for up 10 four months with a 2 Hz sample rate and 20 min averaging interval. We can study flow structure and spectra from as many as four event-triggered recordings of unaveraged samples, each lasting one hour, during periods of intense sediment transport. Acoustic transducer multiplexing permits 24 axes to be interfaced to a single receiving circuit. Electrical reversal of transducers in each axis eliminates zero drift. A deep-sea tripod supports the sensor array rigidly with minimum flow disturbance, yet releases on command for free vehicle recovery.

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