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Microfiber Rotational Dynamics In Turbulence

Vlad Giurgiu, Giuseppe Carlo Alp Caridi, Marco De Paoli, Alfredo Soldati

TU wien, Wien, Austria


We measure the effect of turbulence on spinning and tumbling rates of microfibers in wall-bounded turbulence. The measurements are performed in a turbulent water channel at Reτ 720. Fibres are 1.2 mm long and 10 m 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. In addition, our study reveals that slender, curved fibres behave akin to straight rods, yet their curved shape provides a unique advantage: this curvature allows us to measure all rotational components.

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