Distributed strain sensing in fluid-structure interaction problems using fiber optics

The dynamic interaction between turbulent flows and flexible structures is one of the key challenges in wind engineering, with immediate relevance to the performance and longevity of wind turbine systems. While turbulence can be characterised under controlled conditions, its impact on engineering infrastructure operating machines under realistic turbulent inflows remains insufficiently understood. In wind farms, downstream turbines are immersed in both the wakes of upstream machines, where the incoming flow is characterised by coupled velocity deficits to elevated turbulence intensity, and within the wind farm boundary layer. This exposure creates highly variable loading conditions that accelerate fatigue, limiting operational lifetimes. To address these challenges, we have conducted experiments combining both experimental fluid mechanics and structural health monitoring expertise, to assess and relate the dynamical response of the body to different “favours” of flows. We have conducted experiments using a fibre optic sensing framework based on distributed Rayleigh backscattering (RBS), enabling high-resolution strain measurements, concurrently with experimental flow diagnostics techniques. The approach provides, for the first time, the capability to directly link flow structures to their mechanical imprint on the engineering structure model under analysis. Two complementary experimental setups are studied. The first employs a cantilevered cylinder in a turbulent cross-flow, as a canonical benchmark for bluff-body fluid-structure interaction. Distributed strain measurements along the cylinder surface, taken simultaneously with time resolved particle image velocimetry (PIV), reveal how free-stream turbulence (FST) modifies vortex shedding occuring at the cylinder’s wake, and its respective tip-vortex dynamics, and how these modifications are imprinted onto the cylinder’s fluctuating strain field. Spectral analysis shows how the cylinder acts as a low-pass filter, preferentially coupling with large-scale and low frequency coherent motions in the wake, and that turbulence enhances the correlation between von Kármán vortex shedding and induced loads. Building upon this foundation, a second campaign interrogates the dynamics of a blade of a 1m diameter wind turbine model operating under varying tip-speed ratios in the large section of the 10 × 5 wind tunnel at Imperial College London. The turbine blade was instrumented with a sinusoidally laid fibre-optic network. To allow instrumentation concurrent with the wind turbine’s operation, the system was coupled via a fibre-optic slip-ring, allowing for continuous strain measurement across the rotating blade span. The fibre-optic array captured spanwise variations in bending and twist, distinguishing between root-dominated cyclic loads, mid-span transitional dynamics, and tip-vortex-driven excitation.