Abstract
The objectives of this project were to characterize the performance of Drifting Acoustic Instrumentation SYstems (DAISYs), with an emphasis on flow noise generation, when deployed at depths of up to 15 m and in currents > 2 m/s. In support of this, DAISYs were tested in Admiralty Inlet, WA on July 14, 2022. During these drifts, four DAISY variants were employed:
- Configuration 1: Standard DAISY equipped with a flow shield to suppress flow noise;
- Configuration 2: Standard DAISY with flow shield removed;
- Configuration 3: Standard DAISY without a flow shield and with the hydrophone replaced by an acoustic Doppler velocimeter; and
- Configuration 4: An early prototype DAISY with a smaller surface expression, more compact recording hydrophone system, and no flow shield.
These tests were motivated by a prior comparison performed in relatively shallow and slower moving (< 1.6 m/s) water in the entrance channel to Sequim Bay. During these tests, the early DAISY prototype performed comparably to the standard DAISY, contradicting prior results that showed benefits from using a flow shield.
During the Admiralty Inlet tests, DAISY speed over ground exceeded 3 m/s, providing a robust set of environmental conditions. Flow noise contamination could be as high as 30 dB at frequencies up to 30 Hz. Contrary to expectations, no clear trends were observed with tether length. However, Doppler velocimetry also suggests limited variations in relative velocity and turbulence with depth. By comparing periods of elevated flow noise to hydrophone depth and orientation, we are able to hypothesize two mechanisms by which substantial flow noise can occur for unshielded drifting hydrophones. The first is coherent turbulence that produces rapid changes in hydrophone depth and orientation. The second is vertical shear, which introduces relative velocity between the surface expression and hydrophone. Neither of the mechanisms occurred continuously for all DAISY variants tested, such that unshielded hydrophones could achieve comparable performance to shielded hydrophones during favorable conditions. On the other hand, even when shielded hydrophones encountered turbulence and shear, flow noise levels remained relatively low. These results indicate that flow shields for drifting systems can substantially increase certainty in high-quality data yield at low frequencies (< 100 Hz). Because flow noise is relatively high intensity, it can rapidly mask ambient noise at these frequencies. As such, controlling flow noise to the extent possible is essential to reporting accurate “broadband” acoustic quantities, such as sound pressure level, or pressure spectral densities at relatively low frequencies. If sound pressure levels are substantially inflated by flow noise, this can lead to substantial over-estimates for the acoustic footprints of marine energy converters.
A secondary objective was to investigate approaches to reducing line strum relative to the standard rubber cord on the DAISY by testing the performance of DAISYs with a bare line and a faired line. The faired line proved generally effective at reducing, but not entirely eliminating, strum. Further investigation may be beneficial to identify low-cost approaches for durable fairings on tethers with desirable material properties (e.g., elongation under tension).
Overall, this field test demonstrated the benefits of flow shielding, as well as DAISY functionality in strong currents and their suitability for acoustic characterization of current turbines.