Influence of shear Reynolds number on the dynamics of non-axisymmetric fibres in turbulent channel flow

We investigate experimentally the effect of the shear Reynolds number on the behaviour of long, non-axisymmetric fibres in turbulent channel flow. Fibres are slender, curved and neutrally buoyant particles and their shape ranges from low to moderate curvature. Experiments are performed in the TU Wien Turbulent Water Channel for three values of shear Reynolds number, namely 180, 360 and 720. Time-resolved recordings of the laser-illuminated measurement volume are obtained from four high-speed cameras and used to infer fibres dynamics. With the aid of multiplicative algebraic reconstruction techniques, fibres position and orientation are determined in time. Measurements allow a complete characterization of the fibres dynamics in all the regions of the channel. We observe that the fibres dynamics is strongly influenced by their curvature. Through a comparison between measurements of near-wall dynamics of fibres and near-wall dynamics of flow, we identify a causal relationship between fibre velocity and orientation, and the near-wall turbulence dynamics. Finally, we provide original measurements of the tumbling rate of the fibres, for which we report the influence of fibres curvature, and we confirm previous findings obtained in numerical and experimental works.