Assessment of mid-depth arrays of single beam acoustic doppler velocity sensors to characterise tidal energy sites

Abstract

Accurate characterisation of fluid flow at tidal energy sites is critical for cost effective Tidal Energy Converter (TEC) design in terms of efficiency and survivability. The standard instrumentation in tidal site characterisation has been Diverging acoustic-Beam Doppler Profilers (DBDPs) which remotely measure the flow over a range of scales, resolving up to three velocity vectors. However, they are understood to have several drawbacks particularly in terms of characterising turbulent aspects of the flow. This characterisation is generally based upon a small number of key transient metrics, the accuracy of which directly impacts TEC designs. This work presents an optimisation and performance assessment of newly available Single Beam Doppler Profilers (SBDPs) mounted on a commercial-scale tidal turbine at mid-channel depth in a real operating environment. It was hypothesised that SBDPs would have advantages over DBDPs for site characterisation, in terms of reduced random error, reduced uncertainty in turbulence intensities and the ability to quantify the structure of the turbulent flow. The relationship between random error, sensor orientation and flow speed is quantified for both single and diverging beam sensor types. Random error was found to increase with increasing flow velocity as a power law, the slope of which varies for different sensor orientations. Quantification of noise offers a practical method to correct turbulence metrics. To enable the use of multiple acoustic sensors mounted in close proximity, interference was quantified and mitigation techniques examined. Cross-talk between sensors of the same type were generally shown to bias measurements towards zero. In the presence of alternate types of acoustic sensors, interference caused an increase in standard deviation of velocity results. Implementing a timing offset control mechanism was able to mitigate this effect. This work has achieved a greater understanding of the drivers (spatial separation, inclination angle, pulse power) and effects on measurements of interference along with ambient-noise for users of acoustic instruments. Lessons learned of value to the industry, as site characterisation work intensifies ahead of next generation commercial scale devices, are presented. Mid-channel depth mounted SBDPs were found to have advantages over seabed mounted DBDPs in resolving the key turbulent flow metrics. SBDPs were able to resolve integral length-scales of turbulence that show an anisotropic ratio of scales as predicted from theory and in work at similar sites, while the DBDPs results were similar for all directions. Turbulence intensity measurements were found to be similar after noise correction, with the SBDPs more able to accurately capture the turbulence dissipation rate. This evidence suggests that SBDP arrays present a significant improvement over bottom mounted DBDPs in discerning information about the nature of the turbulent flow, and thus future site characterisation work should consider the use of SBDPs alongside bottom mounted DBDPs for this purpose.