PSI - Issue 80

Francisco J.G. de Oliveira et al. / Procedia Structural Integrity 80 (2026) 1–10

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Author name / Structural Integrity Procedia 00 (2023) 000–000

a)

0.1

a

c

s

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c/R

a

TIP

ROOT

MIDSPAN

0.2

0.8

1.0

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s/R

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bending

twist

fibre optic path

b)

Gears 1:3

Ball bearings

Fibre optic slip ring

Shaft

FC/APC connector

Stator

Rotor

Nacele

Generator

Tower

Fig. 2. a) : Schematic of the fibre-optic sensing layout used, and 3 regions of interest under analysis - ROOT, MIDSPAN and TIP. b) : Schematic of the wind turbine set-up inside of its nacelle, where the slip ring is set in line with the driving shaft of the turbine’s rotor.

2.2. Wind turbine model in turbulent inflow

To extend the fibre-optic methodology to a rotating system, experiments were conducted in the large test section of the 10 × 5 ft wind tunnel at Imperial College London. A horizontal-axis wind turbine of rotor diameter D = 1 . 0 , m was designed using blade element momentum (BEM) theory and optimised to operate at a tip-speed ratio λ = 4. The turbine was mounted vertically with its axis aligned to the incoming flow. One blade was instrumented with a sinusoidal fibre-optic layout on the pressure side, designed to resolve both bending and torsion. The fibre path was defined using a reference grid of pitch a = 40 , mm, producing a helical trajectory of radius r = 12 . 5 , mm along the blade span. This configuration yielded rosettes of strain sensors distributed over several chordwise sections, providing information on both flapwise and edgewise deformation. The fibre was routed through the hub using an in-line optical slip-ring, ensuring uninterrupted transmission of the optical signal under rotation. Data acquisition was performed with the same LUNA ODiSI-B system, providing distributed strain measurements at ∆ s = 2 . 6 , mm resolution across the blade span. Rotor speed was regulated using a MAXON motor operated as a generator (similarly to Bastankhah and Porte´-Agel (2017)), coupled via a 1 : 3 gear train to the drive shaft, allowing control of λ during experiments. Blade dynamics were acquired under operating conditions spanning tip speed ratios ( λ = R ω/ U ∞ , where ω corresponds to the wind turbine’s rotational velocity in rad / s) from λ = 1 to 6 . 5, covering sub-, near-, and super-optimal regimes, at a fixed incoming velocity of U ∞ ≈ 2 . 8 , m / s. The present dataset of distributed blade strain dynamics provides a unique view of the coupled blade system.

3. Impact of FST on a cantilevered cylinder structural dynamics

Engineering components often encounter flow conditions that vary across their span, leading to non-uniform load ing. In the present reference experiment, a cylinder was subjected to distinct free-stream turbulence (FST) inflows to examine how the turbulence characteristics influence its dynamic response. Figure 3 a) displays instantaneous velocity magnitude fields ( U ) fromFOVs C 1 and C 2 for FST cases 1 a and 2 a . These fields reveal that the cylinder’s free end generates a spanwise inhomogeneity in the vortex shedding pattern. The fixed end, by contrast, is the location where