arXiv subjects

Francisco J. G. de Oliveira

Publications and source records attributed to Francisco J. G. de Oliveira.

5 recordsLinked to original sources

arXiv2026

Inter-turbine spacing and flow unsteadiness effects on wake-induced blade dynamics

Wind turbines operating downstream of others in a farm are routinely exposed to waked inflow, with reduced mean velocity and elevated turbulence driving power deficits and additional structural fatigue. The direct effect of wakes on blade-level structural loading remains under-explored experimentally, owing partly to the sparse spatial coverage of conventional point-based strain sensors. Here, we present a wind-tunnel study of wake-induced blade dynamics using two $1\,\mathrm{m}$-diameter turbine models, in which one blade of a downstream turbine ($WT_2$) is instrumented with distributed Rayleigh-backscattering fibre-optic strain sensors, providing spatially continuous strain measurement across the blade span. By changing the relative position of the upstream turbine ($WT_1$) to the downstream, waked turbine $WT_2$ across the streamwise and spanwise extent, we map power output, spanwise strain, and accumulated representative fatigue relevant loading across the wake profile. Full wake impingement suppresses blade loading through the associated velocity deficit, while partial wake overlap generates the strongest load intermittency and highest relative fatigue relevant loading, despite an intermediate power recovery. A combined performance-to-loading metric shows this partial-wake regime offers the least favourable trade-off between energy yield and structural loading. These results show that minimising partial-wake exposure, not only mean velocity deficits, should be a design consideration for wind-farm layout and turbine spacing.

physics.flu-dyn↗

arXiv2026

Cross-correlating blade--wake dynamics for a model wind turbine

Understanding how wakes interact with wind turbine blades under varying operating and inflow conditions is essential for improving fatigue prediction and performance assessment in increasingly dense wind farms. We present an experimental investigation of wake-blade coupling in a model wind turbine, focusing on the role of tip-speed ratio, $λ$, under varying free-stream turbulence conditions. Spatially resolved wake velocity measurements are acquired concurrently with distributed blade strain measurements using Rayleigh backscattering fibre-optic sensing, enabling direct, time-synchronised analysis of fluid-structure interaction across the blade's span. The blades' strain dynamics are strongly governed by $λ$, where variations of the operating condition of the turbine modify the amplitude, coherence, and the temporal/spectral organisation of the blade's structural dynamics, while free-stream turbulence primarily modulates these responses. Instantaneous joint statistics reveal negligible zero-lag dependence between wake velocity and blade strain, motivating a lagged and frequency-resolved analysis. Cycle-averaged cross-correlation and cross-power spectral density analyses demonstrate that wake-induced blade response is spatially localised within the wake shear layers and organised around rotation-coherent frequencies, with the coupling strength peaking at intermediate downstream locations. These results highlight the dominant role of operating condition in shaping wake-mediated blade loading and demonstrate the value of concurrent, spatially resolved flow-structure measurements for resolving blade-exciting flow dynamics in wind-turbine wakes. Furthermore, a consistent negative-lag peak indicates that blade strain fluctuations systematically precede downstream wake velocity fluctuations, suggesting a causal, blade-driven imprint on the wake.

physics.flu-dyn↗

arXiv2026

Wind-turbine wake effects on the rate of accumulation of fatigue damage in overhead conductors

Guidance relating to the safe distance within which overhead conductors (OHCs) should not be built with respect to a wind turbine, and vice versa, varies from country to country. In the United Kingdom the recommendation is that OHCs are not installed within three rotor diameters (3D) of wind turbines due to concerns over wake-induced fatigue. To assess the physical basis for this recommendation, wind-tunnel experiments were conducted using a scaled wind turbine placed upstream of an instrumented conductor, in conditions representative of forested terrain. The conductor was equipped with distributed fibre-optic strain sensing based on Rayleigh backscattering, providing spatially resolved measurements at 2.6 mm resolution. Three conductor heights and four turbine-conductor spacings were tested, together with a no-turbine baseline, while maintaining a constant incident wind speed of 7 m/s. The results identify a critical region near the clamp where mean strain is maximised. Neither mean nor fluctuating strain at this location increased substantially due to the presence of the turbine wake. Strain fluctuations were dominated by aeolian vibration, with the wake increasing vibration amplitude whilst reducing its characteristic frequency. Rainflow counting fatigue analysis shows that damage accumulation depends strongly on conductor height. While fatigue damage rates increase when the conductor is fully immersed in the wake, reduced damage rates are observed for lower heights and closer spacings. These results suggest that, under forested atmospheric conditions, conductor-turbine separations smaller than the current 3D guidance may be feasible provided the conductor is not fully immersed in the turbine wake.

physics.flu-dyn↗

arXiv2025

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

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 $1$ $\mathrm{m}$ 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 $\{\mathrm{FST}, λ\}$ parameter space -- $21$ cases in total. Results reveal that aerodynamic-induced strain fluctuations peak at $λ\approx 3.5$, close to the design tip -speed ratio ($λ_d = 4$), 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 (\textit{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.

physics.flu-dyn↗

arXiv2024

The influence of free-stream turbulence on the fluctuating loads experienced by a cylinder exposed to a turbulent cross-flow

The impact of several $``\text{flavours}"$ of free-stream turbulence (FST) on the structural response of a cantilevered cylinder, subjected to a turbulent cross-flow is investigated. At high enough Reynolds numbers, the cylinder generates a spectrally rich turbulent wake which significantly contributing to the experienced loads. The presence of FST introduces additional complexity through two primary mechanisms: $\textbf{directly}$, by imposing a fluctuating velocity field on the cylinder's surface, and $\textbf{indirectly}$, by altering the vortex shedding dynamics, modifying the experienced loads. We employ concurrent temporally resolved Particle Image Velocimetry (PIV) and distributed strain measurements using Rayleigh backscattering fibre optic sensors (RBS) to instrument the surrounding velocity field and the structural strain respectively. By using various turbulence-generating grids, and manipulating their distance to the cylinder, we assess a broad FST parameter space allowing us to individually explore the influence of transverse integral length scale ($\mathcal{L}_{13}/D$), and turbulence intensity ($TI$) of the FST on the developing load dynamics. The presence of FST enhances the magnitude of the loads acting on the cylinder. This results from a decreased vortex formation length, increased coherence of regular vortex shedding, and energy associated with this flow structure in the near-wake. The cylinder's structural response is mainly driven by the vortex shedding dynamics, and their modification induced by the presence of FST, ie. the indirect effect outweighs the direct effect. From the explored FST parameter space, $TI$ was seen to be the main driver of enhanced loading conditions, presenting a positive correlation with the fluctuating loads magnitude at the root.

physics.flu-dyn↗