Quantifying uncertainty in acoustic measurements of tidal flows using a 'Virtual' Doppler Current Profiler. (1st June 2017)

• Record Type: Journal Article
• Authors: Crossley, George; Alexandre, Armando; Parkinson, Steven; Day, Alexander H.; Smith, Helen C.M.; Ingram, David M.
• Is Part Of: Ocean engineering. Volume 137(2017)
• Page Start: 404
• Page End: 416
• Publication Date: 2017-06-01
• Subjects: Wave -- Current -- Turbulence -- Interaction -- Validation -- Loads; Ocean engineering -- Periodicals; Ocean engineering; Periodicals; 620.4162
• DOI: 10.1016/j.oceaneng.2017.04.015
• Languages: English
• ISSNs: 0029-8018

Abstract

Abstract: Accurate characterisation of flows at tidal sites can enable the developers of tidal stream energy projects to design and model the loads on, and the performance of, tidal energy converters. Acoustic Doppler technology is versatile in the measurement of sea conditions; however, this technology can be limited in its effectiveness at measuring the small-scale kinematic fluctuations caused by waves and turbulence. A Virtual Doppler Current Profiler (VDCP) is used to sample a simulated tidal flow to understand the limitations of this type of measurement instrument whilst recording the small timescale kinematics of waves and turbulence in tidal currents. Results demonstrate the phase dependency of velocity measurements averaged between two acoustic beams and provide a theoretical error for wave and turbulence characteristics sampled under a range of conditions. Spectral moments of the subsurface longitudinal wave orbital velocities recorded by the VDCP can be between 0.1 and 9 times those measured at a point for certain turbulent current conditions, turbulence intensity measurements may vary between 0.2 and 1.5 times the inputted value in low wave conditions and turbulence length scale calculation can also vary hugely dependent on both current and wave conditions. The continuation of this work will enable effective comparison of a linear model for tidal flow kinematics against field measurements from UK tidal site data, and subsequently validate numerical models for the testing of tidal turbines. Highlights: A Virtual Acoustic Doppler Profiler (ADP) is used to sample a simulated flow. The limitations of ADP estimates of velocity, when recording waves and turbulence are presented. Significant errors are found for a range of ADP setup, wave, current and turbulence conditions. Quantification of the uncertainty in velocity measurements at tidal sites enables improved design methods for tidal energy converters.