Abstract
An array of single-beam acoustic Doppler profilers has been developed for the high resolution measurement of three-dimensional tidal flow velocities and subsequently tested in an energetic tidal site. This configuration has been developed to increase spatial resolution of velocity measurements in comparison to conventional acoustic Doppler profilers (ADPs) which characteristically use divergent acoustic beams emanating from a single instrument. This is achieved using geometrically convergent acoustic beams creating a sample volume at the focal point of 0.03 m3. Away from the focal point, the array is also able to simultaneously reconstruct three-dimensional velocity components in a profile throughout the water column, and is referred to herein as a convergent-beam acoustic Doppler profiler (C-ADP). Mid-depth profiling is achieved through integration of the sensor platform with the operational commercial-scale Alstom 1 MW DeepGen-IV Tidal Turbine deployed at the European Marine Energy Center, Orkney Isles, UK. This proof-of-concept paper outlines the C-ADP system configuration and comparison to measurements provided by co-installed reference instrumentation.Comparison of C-ADP to standard divergent ADP (D-ADP) velocity measurements reveals a mean difference of 8 mm s−1, standard deviation of 18 mm s−1, and an order of magnitude reduction in realisable length scale. C-ADP focal point measurements compared to a proximal single-beam reference show peak cross-correlation coefficient of 0.96 over 4.0 s averaging period and a 47% reduction in Doppler noise.The dual functionality of the C-ADP as a profiling instrument with a high resolution focal point make this configuration a unique and valuable advancement in underwater velocimetry enabling improved quantification of flow turbulence. Since waves are simultaneously measured via profiled velocities, pressure measurements and surface detection, it is expected that derivatives of this system will be a powerful tool in wave-current interaction studies.
Highlights
Improved understanding of the dynamics of tidal currents and oceanic waves and their complex interaction is a prerequisite for an economically viable marine hydrokinetic turbine industry: complex velocity fields drive structural loads which affect device design, reliability and energy conversion rate
Because the velocity measurement of each beam is calculated from the Doppler shift the velocity component is measured in the direction of the beam itself
This paper presents an alternative configuration of geometrically convergent acoustic beams, in an effort to combine the desirable properties of the D-acoustic Doppler profilers (ADPs) and Acoustic Doppler velocimeters (ADVs) instrument to achieve increased spatio-temporal resolution of velocity measurements within a challenging marine environment
Summary
Improved understanding of the dynamics of tidal currents and oceanic waves and their complex interaction is a prerequisite for an economically viable marine hydrokinetic turbine industry: complex velocity fields drive structural loads which affect device design, reliability and energy conversion rate. Characterisation of the turbulent flow is limited by existing velocity measurement technology. In these typically heterogeneous flows, velocimetry using divergent-beam acoustic Doppler profilers (D-ADPs) is unable to capture instantaneous three-dimensional velocity information at the necessary spatial scales. Acoustic Doppler velocimetry techniques, geometrically divergent acoustic beam configurations, are widely used in the field measurement of offshore flow velocities due to the relative ease of configuration and installation, unobtrusive flow measurements, as well as the ability to sample throughout the water column. While a variety of beam configurations exist, in order to deduce a three-dimensional velocity measurement, these acoustic beams must be transmitted in at least three directions [6]. Because the velocity measurement of each beam is calculated from the Doppler shift (resulting from the scattering of sound by suspended particles in the water) the velocity component is measured in the direction of the beam itself
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