PSI - Issue 80

Francisco J.G. de Oliveira et al. / Procedia Structural Integrity 80 (2026) 1–10 Author name / Structural Integrity Procedia 00 (2023) 000–000

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The results demonstrate that FST strengthens the contribution of vortex shedding to the structural dynamics. While Kankanwadi and Buxton (2023) identified the turbulence length scale as the key parameter influencing near-wake entrainment, our findings indicate that turbulence intensity is the dominant factor controlling the amplification of the structural response, especially associated with the increased relevance of vortex shedding on the structural response.

4. Retrieving aerodynamics from strain measurements at blade level

The fibre-optic instrumentation of the wind turbine blade provides, for the first time, spanwise-resolved measure ments of structural response under rotation and turbulent inflow. The D = 1 m model turbine was operated in the Imperial College 10 × 5 wind tunnel across a range of tip-speed ratios λ = 1 − 6 . 5, subjected to a constant inflow wind speed of U ∞ = 2 . 8m / s, and TI = 3 . 7%. A single blade of the 3-bladed rotor was instrumented, with a sinusoidal Rayleigh backscattering fibre-optic layout. The chosen layout allows us to decompose the acquired strain in the 2 relevant flapwise and edgewise components. The fibre optic sensors were routed through an optical slip ring at the hub, enabling continuous acquisition of strain distributions along the blade span, while the turbine operated.

Fig. 7. Spanwise time-averaged flapwise and edgewise strain distribution for representative λ .

Figure 7 presents the spanwise distributions of time-averaged flapwise (left panels) and edgewise (right panels) strain for representative tip-speed ratios λ = 2 , 4 , and 6 . 5. The measurements provide direct insight into how the loading on the blade evolves with operating condition. At the below-optimal tip-speed ratio ( λ = 2), the blade experiences large root bending loads, and the strain remains relatively uniform along the span. The absence of strong gradients reflects a loading state dominated by quasi-rigid deflection, with aerodynamic contributions comparatively weak relative to inertial and centrifugal e ff ects. At the design condition ( λ = 4), the loading redistributes along the span: mid-span regions show a clear increase in flapwise strain, indicating enhanced aerodynamic forcing. In the edgewise component, distinct features emerge in the outer span, aligned with tip-vortex activity. This redistribution highlights the balance between aerodynamic lift, centrifugal sti ff ening, and root bending, resulting in a more heterogeneous structural response than at low λ . At the above-optimal operating point ( λ = 6 . 5), the overall magnitude of both flapwise and edgewise strain de creases compared to the design condition, consistent with reduced aerodynamic e ffi ciency at high λ . However, lo calised tip-region signatures persist, reflecting the sustained influence of tip-vortex dynamics despite the overall drop in load levels. The edgewise strain distribution, in particular, retains spanwise oscillatory features near the tip, sug gesting that tip-vortex induced loading remains a key mechanism for structural excitation beyond optimum.