Effect of free-stream turbulence on the hydrodynamic performance and wake structure of an H-Darrieus tidal turbine
Date
2025
Authors
Magas, Chad
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Abstract
An increasing awareness of the impact of anthropogenic pollution on climate and ecosystems has driven innovation and research into renewable energy technologies. Hydrokinetic energy harvesting of tidal events is among the most promising of these renewable technologies due to the predictability of tides. The H-Darrieus turbine stands out as one of the most capable devices for this energy conversion. Despite continued research efforts, the effects of free-stream turbulence on H-Darrieus turbines still pose substantial challenges in both the research and implementation of these devices.
The present work investigates the effect of free-stream turbulence on the hydrodynamic performance and wake structure of an H-Darrieus tidal turbine. To perform this investigation, experiments were conducted in the recirculating water channel inside the University of Victoria’s Fluid Mechanics Laboratory. The turbine used was a 1/10th scale H-Darrieus turbine, operating at the diameter-based Reynolds number 0.5×10^6. Fractal grids were installed upstream of the turbine to generate three distinct free-stream inflow turbulence levels: 1.4% (baseline), 4.7% (intermediate), and 10.4% (high). The performance of the turbine was evaluated at each turbulent interval across the tip-speed ratio (TSR) range of 1.0 ≤ TSR ≤ 3.4. Quantifying the performance was achieved through continuous measurement of the shaft torque and azimuthal position data. The wake structure of the turbine was characterised at TSR = 2.65 using particle image velocimetry to obtain ensemble- and phase-averaged velocity measurements.
Phase-resolved performance data suggest that an increase in free-stream turbulence delays the onset of stall and accelerates the reattachment of the boundary layer, impacting critical azimuthal angles at which the turbine rotor interacts with the free-stream. These results demonstrate an increase in quasi-periodicity of shaft torque with increasing free-stream turbulence, leading to greater cycle-to-cycle azimuthal variance in vortex shedding.
The wake flow fields obtained at TSR = 2.65 indicate that elevated levels of free-stream turbulence decrease the transition distance from near- to far-wake. This decrease in wake coherence is a result of increased quasi-periodicity in the vortex shedding. Furthermore, increased free-stream turbulence was found to increase the streamwise velocity deficit, leading to a delayed wake recovery. This decrease in the total streamwise momentum recovery is suggested to be the cause of a reduction in the out-of-plane transportation, representing a faster breakdown and dissipation of blade tip-vortices.
These results offer direct implications in the context of tidal turbine design and deployment. The increase in quasi-periodicity of the turbine with elevated turbulence introduces non-periodic loading as well as a decreased unloading period onto the turbine blades. This increase in load and decrease in loading time may require a more robust turbine design and system integration strategy, depending on the free-stream turbulence characteristics of the tidal environment. Additionally, the change in momentum transfer rate in the wake of the turbine will require additional consideration for optimal turbine array spacing.
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Keywords
H-Darrieus turbine, Cross-flow turbine, Wake structure, Power extraction performance, Turbulence, Particle image velocimetry, Experimental