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

TL;DR

This work addresses how free-stream turbulence, quantified by $TI$ and $L_{13}/D$, modulates flow-induced loads on a bluff-body cantilevered cylinder in turbulent cross-flow. It uses a concurrent measurement framework combining temporally resolved $2$D-$PIV$ and distributed Rayleigh backscattering sensing to map wake dynamics to local strain over the cylinder, across nine FST flavours. The results show that increasing $TI$ lowers vortex formation length $x^*/D$, raises near-wake energy, and enhances spanwise coherence of regular vortex shedding, leading to larger root bending loads; the indirect wake modification dominates the structural response, with vortex shedding identified as the main contributing coherent structure. The findings provide practical insights for design against FST in bluff-body applications and demonstrate the value of integrated flow–structure diagnostics with $OMD$-based modal analysis to attribute loads to specific wake features.

Abstract

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.

Figures (15)

• Figure 1: Experimental schematic layout: (a)- fibre optic sensing path, fields of view (FOV) captured and representation of the main flow events over the cylinder. The used Cartesian space of coordinates is represented in the figure, where $y$ corresponds to the spanwise direction of the cylinder, $x$ to the streamwise and $z$ to the transverse direction of the flow.
• Figure 2: (a): FST $\{\mathcal{L}_{13}/D , TI\%\}$ parameter space tested. Groups are defined based on their $TI$, to explore within each group the effect of increasing $\mathcal{L}_{13}$. (b): PIV snapshots captured in FOV A for the FST conditions described by cases $1a$, no FST, and $3a$, high $TI$ and medium $\mathcal{L}_{13}$. The black circle corresponds to the position of the cylinder during the experimental tests, and the grey region to the shadow originating from the positioning of the cylinder with respect to the laser sheet.
• Figure 3: (a): Time-averaged strain ($\overline{\varepsilon}$)() and reconstructed deflection field ($\overline{\delta}$) ( ) obtained by $\theta_f^{\alpha}$, $\alpha \in [45, -45, 135, -135]^{\circ}$ respectively represented by , , and . The waviness in the strain distribution is related to the non-uniformity wall thickness of the cylinder, due to its manufacturing process. (b): Averaged normalised tip deflection ($\delta_{\text{tip}}/L$) for each FST case.
• Figure 4: Fluctuating root bending stresses, characterised by $\gamma$, with respect to $TI$a) and $\mathcal{L}_{13}/D$b) content in the free-stream. $m$ corresponds to the slope of the linear best fit of the evolution of $\gamma$ with $TI$.
• Figure 5: a) and b): vortex formation length ($x^{*}/D \rightarrow max(rms(\mathbf{u}_3^{\prime}(-1<z/D<1)))$) variation of the wake of the cylinder, for each of the FST conditions tested, and each FOV interrogated (FOV A $\rightarrow$a), FOV B $\rightarrow$b)).