A low-cost, high-fidelity converging-beam Doppler instrument for measuring velocity and turbulence at tidal energy sites

Abstract

Traditional Acoustic Doppler Current Profiler (ADCP) instruments measure marine flow velocities along diverging acoustic beams. This allows estimation of approximate mean velocity and turbulence at a point between the beams. Alternatively, a converging beam instrument can measure velocity components at a single point and hence provide a higher quality measurement. Here we show a new design of instrument with converging beams together with some preliminary flow measurement results.

The instrument is constructed around a triangular frame with Doppler transceivers on the ends of cable stayed arms. It is designed to be installed and recovered from the seabed without use of a crane vessel. This is achieved by a pressurised air buoyancy system. The system can be slipway launched with a boat trailer and flat packed for transport on the same trailer. The system performed well for three test deployments; however, measurements of the seabed stability of the frame showed undesirable flexing of one arm when it was positioned perpendicular to the main flow direction.

A traditional ADCP was located on the frame and the two instruments were operated in burst mode, with each instrument measuring alternately every 20 minutes. Results for velocity and turbulent kinetic energy at the same depth are reported for both instruments and compared. A 2D oceanographic model of the deployment site is used as an additional point of comparison to illuminate some differences in the mean flow velocity observations of both instruments.

This instrument will add significantly to measurement capabilities at tidal stream turbine deployment locations. Improved turbulence measurements will give better understanding of turbine loading and hence improve reliability of these systems. The unit can also be used to deploy different types of sensors for oceanographic measurements.