Rotational Dynamics of Rod Particles in Fluid Flows

The primary aim of this research is to study the dynamics of rods in fluid flows, quantify the alignment of rods with the flow and the effects of alignment on the rotation rates. We perform experimental measurements of the rotation of rods in three-dimensional turbulence and resolve the rotational properties. In a 2D chaotic flow we measured the translational and rotational dynamics of rods along with the velocity of the carrier flow. Rods are strongly aligned with the stretching direction of the Cauchy-Green deformation tensor. We report the first three-dimensional measurements of the rotational dynamics of rod-like particles as they are advected in a turbulent fluid flow. Tracer rods preferentially sample the flow since their orientations become correlated with the velocity gradient tensor. The probability distribution of the mean square rotation rate has a long tail which implies the presence of rare events with large rotation rates. Rotation of particles is controlled by small scales of turbulence that are nearly universal, these measurements provide a rich system where experiments can be directly compared with theory and simulations. In another set of experiments we measured the rotational statistics of neutrally buoyant rods with lengths 2.8 < l/η < 72.9, where η is the Kolmogorov length scale, in turbulence and quantify how their rotation rate depends on length. The mean square rotation rate of rods decreases as the length of the rods increases and for lengths in the inertial range. We derive an scaling of l^-4/3 for the mean square rotation rate and show that experimental measurements approach this scaling law. In comparison with the randomly oriented rods we see that all rod lengths develop alignment with the velocity gradient of the flow at the length of the rods. We have also measured the correlation time of the Lagrangian autocorrelation of rod rotation rate and find that the correlation time scales as the turn over time of eddies of the size of the rod. Measuring the rotational dynamics of single long rods provides a new way to access the spatial structure of the flow at different length scales.