Experimental measurement of spinning rates of Kolmogorov scale slender fibres

We measure the effect of curvature of slender fibres on spinning rates in wall-bounded turbulence. The experiments are done in the TU Wien Turbulent Water Channel at $Re_{\tau}$ $180, \ 360$, and $720$. Fibre lengths range from below $1$ up to $20$ Kolmogorov lengths. Their aspect ratio ranges from $40$ to $120$. In these flow conditions they are neutrally buoyant, inertia-less, and rigid.

The fibres are imaged by six high-speed cameras across the full channel half-height. An established technique of fibre reconstruction and tracking is employed (Alipour et al. 2021). The optical magnification is improved achieving higher 3D reconstruction resolution (up to $100$ voxel per fibre length). The curved shape enables spinning rate measurements. It allows the definition of the fibre containing plane and tracking its change in orientation. We provide evidence of the complete fibre rotation rate. We observed that with increasing curvature spinning decreases. The ratio of spinning to tumbling rate variances is found to be higher than unity. The curved shape couples tumbling and spinning motions. These results are novel because previous experiments are limited to measuring rotation rates of longer straight fibres in homogeneous isotropic turbulence or restricted to tumbling rates only.