Spinning and tumbling of long fibers in isotropic turbulence

TL;DR

This study investigates the rotational behavior of long fibers in isotropic turbulence, revealing how their spinning and tumbling are influenced by flow structures and scale-dependent dynamics.

Contribution

It provides the first simultaneous measurements of spinning and tumbling of long fibers in turbulence, highlighting their alignment with vortex filaments and the scaling laws governing their rotation.

Findings

Spinning rate variance scales as d^(-4/3) with fiber diameter.

Long fibers tend to align with vortex filaments.

Coriolis effects significantly influence fiber rotation.

Abstract

We simultaneously measure both spinning and tumbling components of rotation for long near-neutrally buoyant fibers in homogeneous and isotropic turbulence. The lengths and diameters of the measured fibers extend to several orders of the Kolmogorov length of the surrounding turbulent flow. Our measurements show that the variance of the spinning rate follows a -4/3 power law scaling with the fiber diameter ($d$) and is always larger than the variance of the tumbling rate. This behavior surprisingly resembles that observed previously for sub-Kolmogorov fibers. The general picture that emerges from this study is that long fibers preferentially align with vortex filaments that can be as long as the integral length of turbulence. We compute the Lagrangian time scale and the distribution of both tumbling and spinning that supports this outlook. Our measurements also allow us to quantify the importance of the Coriolis term on the rotational dynamics of fibers in turbulent flows.