Messung der Winkelgeschwindigkeit von schlanken, gekrümmten Fasern der Kolmogorov Größe

Abstract

We measure the complete angular velocity of slender micro-plastic curved fibres in wall- bounded turbulence. The experiments are done in the TU Wien Turbulent Water Channel at shear Reynolds number 720. Fibre aspect ratio is 120 and their length is 8 Kolmogorov lengths. They are non-deformable, torsion-less, nearly neutrally buoyant, and have negligible inertia in these flow conditions. This study is original because previous experiments are limited to meas- uring rotation rates of large straight rods in homogeneous isotropic turbulence or restricted to tumbling rates only in turbulent channel flow. A 3D time-resolved particle image velocimetry system image the fibres in a close wall region. An established technique of fibre reconstruction and tracking is employed (Alipour et. al 2021) and extended. It consists of four steps: (i) the three-dimensional shape of each fibre is reconstructed with a tomographic algorithm, (ii) track- ing, (iii) modelling of each fibre as a 2nd order polynomial, and (iv) determination of translation and rotation. The unique orientation over time is inferred and used to compute spinning and tumbling rates. We validate our in-house fibre code by generating synthetic fibres with known motion and perform a parameter sensitivity analysis. The uncertainty of the measured spinning rate is found to depend on the length of the minor axis of the curved fibre and its rotational rate. Finally, the ratio of mean spinning to tumbling rate was found higher than unity.