Measurements of spinning and tumbling rates of Kolmogorov scale micro-plastic fibres

We measure the effect of the wall-normal location of micro-plastic fibres on spinning and tumbling rates in wall-bounded turbulence. The measurements are performed in a turbulent water channel at shear Reynolds number of 720. Fibres are 1.2mm long and 10μm in diameter (aspect ratio 120). Their length ranges from 4 to 12 Kolmogorov length scales. They are neutrally buoyant, inertial-less, and rigid in these flow conditions.

Six high-speed cameras image the fibres at the channel centre and in a near-wall region. We employ and further refine a technique of tomographic fibre reconstruction and tracking. Their curved shape is used to define a fibre-fixed reference frame and measure its time-resolved orientation. Thus measurements of tumbling and spinning rates are enabled. We provide a discussion about the uncertainty on the rotation rates based on their shape and angular displacement between time-steps. Based on converged statistics, we observed that the mean and mean square spinning are higher than tumbling rates at both channel centre and near-wall region. Our results are original, because previous measurements are restricted to rotation rates of longer rods in homogeneous isotropic turbulence or to tumbling rates only.