Accession Number : ADA333244

Title :   Development of the BASS Rake Acoustic Current Sensor: Measuring Velocity in the Continental Shelf Wave Bottom Boundary Layer

Descriptive Note : Doctoral thesis

Corporate Author : MASSACHUSETTS INST OF TECH CAMBRIDGE

Personal Author(s) : Morrison, Archie T., III

PDF Url : ADA333244

Report Date : JUN 1997

Pagination or Media Count : 357

Abstract : Surface swell over the continental shelf generates a sheet of oscillatory shear flow at the base of the water column, the continental shelf wave bottom boundary layer. The short periods of surface swell sharply limit the thickness of the wave boundary layer, confining it to a thin region below an oscillatory, but essentially irrotational, core. For a wide range of shelf conditions, the vertical extent of the wave boundary layer does not exceed 2.5 cm and is commonly less. The extreme narrowness of this boundary layer is responsible for high levels of bottom stress and turbulent dissipation. Even in relatively mild sea states, the wave induced bottom shear stress can be sufficient to initiate sediment motion. The wave bottom boundary layer plays an important role in the processes of sediment entrainment and transport on the continental margins. This thesis documents the development, testing, and field use of a new instrument, the BASS Rake, designed to measure velocity profiles in the wave boundary layer. The mechanical design supports multiple measurement levels with millimeter vertical spacing. The mechanical design is integrated with an electronic interface designed to permit flexible acquisition of a suite of horizontal and vertical velocity measurements without sacrificing the electronic characteristics necessary for high measurement accuracy. The effects of velocity averaging over the sample volume are calculated with a model of acoustic propagation in a scattering medium appropriate to the scales of a single differential travel time axis. A simpler parametric model of the averaging process is then developed and used to specify the transducer characteristics necessary to image the wave boundary layer on the continental shelf.

Descriptors :   *OCEAN WAVES, *CONTINENTAL SHELVES, *ACOUSTIC DETECTORS, OCEAN CURRENTS, OCEAN SURFACE, THESES, TURBULENCE, TURBULENT FLOW, TURBULENT BOUNDARY LAYER, SEDIMENT TRANSPORT, FLOW FIELDS, SHEAR STRESSES, ACOUSTIC ARRAYS, SOUND TRANSMISSION, SEA STATES, OCEAN MODELS, SHEAR FLOW, BOUNDARY LAYER FLOW, LASER VELOCIMETERS.

Subject Categories : Physical and Dynamic Oceanography
      Acoustic Detection and Detectors

Distribution Statement : APPROVED FOR PUBLIC RELEASE