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Accueil > Actualités > Séminaires > Séminaires 2024

Mardi 5 Novembre 11h00 - LEGI Salle K118

Alfredo Soldati, TU Wien

Preferential orientation and rotation of microfibers and their connection with small-scale turbulence dynamics

The straining and rotation associated with small-scale turbulent dynamics are characterized and controlled by vortex tubes of strong rotation intensity – high enstrophy regions – and sheets of strong straining motion – high dissipation regions. Via the nature of the local velocity gradient, these regions can influence the dynamics of small-scale particles. For instance, inertial spherical particles are ejected from vortical regions because of centrifugal forces and concentrate in strain-dominated ones, whereas inertialess anisotropic elongated particles are known to sample preferentially the coherent, intense, tubular vortices [1]. In addition, since these particles are also known to align their principal axis in the direction of vorticity, measurement of their rotation rate can provide information of the dynamics of small-scale turbulence [2]. In this talk, we will briefly review the physics of elongated particles in turbulence, and we will describe recent experiments performed in the TU Wien Turbulent Water Channel [3]. In detail, we will exploit the tendency of elongated particles to preferentially concentrate in coherent rotational structures and to preferentially align with the vorticity, and we will perform original optical Lagrangian measurements of their rotation rates around their longitudinal axis – spinning rate – and transversal axes – tumbling rates. We use elongated, large aspect ratio, and mildly curved plastic fibers, which are slightly longer than the Kolmogorov length scale. A bonus of the mildly curved shape is that it gives these particles a uniquely identifiable three dimensional orientation that can provide us with a set of homogeneous data on their full rotation rates. These data, explained in connection with the general features of turbulence, can give us instruments to determine motion, dispersion, and sedimentation rate of anisotropic particles. Such results will contribute to predicting the dynamics of small anisotropic microplastics polluting the oceans.

References
[1] J.R. Picardo, R. Singh, S.S. Ray, and D. Vincenzi Phil. Trans. R. Soc. A 378, 20190405 (2019).
[2] S. Brizzolara, M.E. Rosti, S. Olivieri, L. Brandt, M. Holzner, A. Mazzino, Phys. Rev. X 11, 031060 (2021).
[3] V. Giurgiu, G. Caridi, M. Alipour, M. De Paoli, and A Soldati, Rev. Sci. Instrum. 94, 095101 (2023).


Contact Nathanaël Machicoane for more information or to schedule a discussion with the seminar speaker.