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

TL;DR

This study investigates how different types of free-stream turbulence affect the fluctuating loads on a cylinder in turbulent cross-flow, revealing that turbulence intensity significantly amplifies load fluctuations mainly through altered vortex shedding.

Contribution

It provides a detailed experimental analysis of how free-stream turbulence parameters influence vortex shedding and load dynamics on a cylinder, highlighting the dominant role of turbulence intensity.

Findings

FST increases load magnitudes on the cylinder.

Turbulence intensity correlates positively with load fluctuations.

FST modifies vortex shedding dynamics, reducing vortex formation length.

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.