Influence of Reynolds number on the dynamics of rigid, slender and non-axisymmetric fibres in channel flow turbulence

Abstract

We investigate experimentally the dynamics of non-axisymmetric fibres in channel flow turbulence, focusing specifically on the importance of the fibre size relative to the flow scales. To this aim, we maintain the same physical size of the fibres and we increase the shear Reynolds number. Experiments are performed in the TU Wien Turbulent Water Channel for three values of shear Reynolds number, namely 180, 360 and 720. Fibres are slender – length to diameter ratio of 120 – rigid, curved and neutrally buoyant particles and their shape ranges from low curvature – almost straight fibres – to moderate curvature. In all cases, fibre size remains small compared with the channel height (${\leqslant }1.5\,\%$). Three-dimensional and time-resolved recordings of the laser-illuminated measurement region are obtained from four high-speed cameras and used to infer fibre dynamics. With the aid of multiplicative algebraic reconstruction techniques, fibre position, orientation, velocity and rotation rates are determined. Our measurements span over the half-channel height, from wall to centre, and allow a complete characterisation of the fibre dynamics in all regions of the flow. Specifically, we measure fibre preferential distribution and orientation. We observe that the fibre dynamics is always influenced by their curvature. Through a comparison between measurements of the near-wall dynamics of the fibres and the near-wall dynamics of the flow, we identify a causal relationship between fibre velocity and orientation, and the near-wall turbulence dynamics. Finally, we have been able to provide original measurements of the tumbling rate of the fibres, for which we report the influence of fibre curvature. We underline that our measurements confirm previous findings obtained in numerical and experimental works.

Effect of Shape and Size on the Transport of Floating Particles on the Free Surface in a Natural Stream

“…In such cases, particle imaging may require the use of uncrewed aerial vehicles, which have been successfully utilized to characterize natural flows (Blois et al, 2016;Liu et al, 2021). Given the variety of debris types found in water streams, the behavior of irregularly shaped particles is highly relevant, as highlighted by recent studies of non-floating fibers in turbulent channel flows (Alipour et al, 2021(Alipour et al, , 2022. Also, the range of particle properties should be expanded beyond shape, size, and density: deformability and brittleness have recently been investigated in laboratory studies and are especially relevant to plastic pollution (Brouzet et al, 2014(Brouzet et al, , 2021.…”

Microfiber Rotational Dynamics In Turbulence

“…was used to reconstruct and track each fibre from the six 2D images. These measurements were improved with respect to Alipour et al (2021Alipour et al ( , 2022 through the addition of two extra viewing directions, which increase the reconstruction accuracy [Giurgiu et al (2023); Elsinga et al (2006)] and by doubling the resolution, which reduces the orientation uncertainty [Caridi et al (2023)].…”

“…Experiments by Ref. Alipour et al (2021Alipour et al ( , 2022; Kuperman et al (2019) report only tumbling rates of slender, curved fibres at the Kolmogorov length scale. However, the complete rotation rate of these particles remained unknown.…”