Measurements of the wake from a floating tidal energy platform

Abstract

Wake measurements are critical for quantifying the hydrodynamic impacts of turbine presence and tidal energy extraction on the tidal flow. Turbine wakes are typically assessed using numerical models and controlled laboratory experiments, with only a few field studies available for the wakes of full-scale operating tidal turbines. In this investigation we present field observations of the combined wake generated by the four-turbine array mounted onboard Sustainable Marine Energy Canada PLAT-I 4.63. Measurements were conducted downstream of the platform in Grand Passage, a tidal channel in the southwest of the Bay of Fundy in eastern Canada in October 2020. Velocity data were obtained by a suite of mobile Acoustic Doppler Current Profilers (ADCP), both vessel-mounted and free-drifting. Data were collected during ebb and flood tides (and therefore with time-varying inflow velocity), and under different turbine operating conditions. The collected data were organized according to the turbine inflow velocity for ebb and flood tide. For each tide, the wake and undisturbed flow regions to the sides of the wake were identified. Vertical profiles of velocity in the wake were compared to inflow velocity measured by a current-meter onboard PLAT-I and with measurements in the undisturbed flow to the sides of PLAT-I wake. In all measurements the PLAT-I wake manifests as a reduction in flow speed at the depths spanned by the turbine rotors. The reduction is maximum near the platform for both ebb and flood. For flood, velocity profiles vertically mix less than 5 effective diameters downstream of the array, but velocities remain slower compared to the flow outside of the wake. Flow speed increases downstream, recovering approximately about 20 effective diameters from the platform. For ebb, the velocity reduction persists farther downstream compared to flood, there is less vertical mixing, and the wake shape is still present beyond 10 effective diameters downstream of the platform. Increased turbulence is also observed downstream of the platform, which recovers to levels similar to those of the surrounding undisturbed flow about 10 effective diameters downstream of the turbine for both ebb and flood. Comparisons of results between the two measurements approaches, and between the wake of clean and bio-fouled turbines are also explored.