PNNL

Abstract

The deployment of acoustic Doppler velocimeters (ADVs) on tethered instrumentation platforms has recently been investigated for the characterization of turbulence for the emerging tidal energy industry. A variety of instrumentation platforms have been deployed as part of this work including the streamlined StableMoor buoy (SMB), the Tidal Turbulence Mooring (TTM) system based on a conventional 0.9 m spherical buoy, and a 100 lb sounding weight. The first two systems are bottom mounted moorings and the latter is deployed from a research vessel. The motion-induced velocities at the ADV head on each platform type and instrument configuration are discussed in the context of flow energies relevant to tidal energy and with the objective of reducing motion contamination of measurements. The SMB with a single ADV head mounted on the nose provided the most stable platform for the measurement of tidal turbulence in the inertial sub-range for flow speeds exceeding 1.0 m/s. The modification of the SMB with a transverse wing configuration for multiple ADVs showed a similar frequency response to the nose configuration but with large contamination in the vertical direction as a result of platform roll. While the TTM provided a relatively stable configuration at low frequencies of motion, the motion-induced velocity at the ADV head became significant at frequencies above f = 1 Hz. The sounding weight measurements showed the greatest motion at the ADV head but are likely to be influenced by both prop-wash and vessel motion.