Abstract
Oscillating hydrofoils were installed in a water tunnel as a surrogate model for a hydrokinetic cross-flow tidal turbine, enabling the study of the effect of flexible blades on the performance of those devices with high ecological potential. The study focuses on a single tip-speed ratio (equal to 2), the key non-dimensional parameter describing the operating point, and solidity (equal to 1.5), quantifying the robustness of the turbine shape. Both parameters are standard values for cross-flow tidal turbines. Those lead to highly dynamic characteristics in the flow field dominated by dynamic stall. The flow field is investigated at the blade level using high-speed particle image velocimetry measurements. Strong fluid–structure interactions lead to significant structural deformations and highly modified flow fields. The flexibility of the blades is shown to significantly reduce the duration of the periodic stall regime; this observation is achieved through systematic comparison of the flow field, with a quantitative evaluation of the degree of chaotic changes in the wake. In this manner, the study provides insights into the mechanisms of the passive flow control achieved through blade flexibility in cross-flow turbines.
Highlights
Hydrokinetic energy in oceanic currents is an energy resource with large potential that remains largely unexploited
The present study focuses on the flow field surrounding a single cross-flow tidal turbines (CFTTs) blade, as investigated on a pitching flexible hydrofoil by means of high-speed particle image velocimetry measurements
Starting with the rigid hydrofoil, the reference for a conventional CFTT, deep dynamic stall characteristics are found: the flow stays attached while passing through the static stall angle of α = 15°
Summary
Hydrokinetic energy in oceanic currents is an energy resource with large potential that remains largely unexploited. Sustainable exploitation technologies for marine and tidal streams are the focus of recent research, driven by the aim to reduce climate change and promote a greenhouse gas-neutral production of electrical energy. In this context, hydrokinetic turbines are of particular interest, as they might solve the most critical issues for a successful application in ocean energy engineering: sustainability and cost competitiveness against on-shore technologies. Hydrokinetic vertical-axis or cross-flow tidal turbines (CFTTs)—the scope of this study—seem to be advantageous compared to classical axial, called horizontal-axis turbines They are predestined for array installations because of their rectangular crosssection and better fit in shallow water installations. They are in general of simple construction and operate independently of the stream direction, which is an important advantage for applications in tidal streams, featuring frequent variations in flow speed and direction [4]
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