The effect of tip-speed ratio and free-stream turbulence on the coupled wind turbine blade/wake dynamics

Abstract. Wind turbines operating within wind farms experience complex aerodynamic loading arising from the interplay between wake-induced velocity deficits, enhanced turbulence, and varying operational conditions. Understanding the relationship between the blade's structural response to the different operating regimes and flow structures generated in the turbine's wake is critical for predicting fatigue damage and optimizing turbine performance. In this work, we implement a novel technique, allowing us to simultaneously measure spatially distributed blade strain and wake dynamics for a model wind turbine under controlled free-stream turbulence (FST) and tip-speed ratio (λ) conditions. A 1 m diameter three-bladed rotor was instrumented with distributed Rayleigh backscattering fibre-optic sensors, while synchronised hot-wire anemometry captured wake evolution up to 4 rotor diameters downstream. Experiments were conducted covering a wide {FST, λ} parameter space – 21 cases in total. Results reveal that aerodynamic-induced strain fluctuations peak at λ ≈ 3.5, close to the design tip -speed ratio (λ = 4), with the blade's tip experiencing a contribution from the aerodynamically-driven strain fluctuations of up to 75 % of the total fluctuating strain at design conditions. Spectral analysis shows frequency-selective coupling between wake flow structures and the blade response, dominated by flow structures dynamically related to the rotor's rotating frequency (eg. tip vortex structure). The novel experimental methodology and results establish a data-driven foundation for future aeroelastic models' validation, and fatigue-informed control strategies.