Measurements of spinning and tumbling rates of micro-plastic fibres

We perform measurements to assess the influence of the wall-normal position of micro-plastic fibres on their spinning and tumbling rates in wall-bounded turbulence. The exper-iments are carried out in a turbulent water channel at a Shear Reynolds number of 720.The used fibres are curved, 1.2mm long, and 10μm in diameter (aspect ratio 120). Theirlength ranges between 4 and 12 Kolmogorov length scales. In the generated flow condi-tions they are inertial-less, neutrally buoyant, and undeformable. We observe their motionwith six high-speed cameras focused in the near-wall region and channel centre. We employand improve upon an established methodology involving the tomographic reconstruction ofeach fibre and subsequent tracking. Leveraging their curved shape, we uniquely identify thetemporal evolution of their orientation, enabling measurements of spinning and tumblingrates. We discuss the uncertainty on the rotation rates based on their shape and angulardisplacement between time-steps. Analysis of converged statistics revealed that the meanand mean square spinning are higher than tumbling rates at both channel centre and near-wall region. These results are novel, considering that previous experiments are restricted tomeasurements of rotation rates of longer straight fibres in homogeneous isotropic turbulenceor to tumbling rates only.