We present simultaneous experimental measurements of the dynamics of anisotropic particles transported by a turbulent flow and the velocity gradient tensor of the flow surrounding them. We track both rod-shaped particles and small spherical flow tracers using stereoscopic particle tracking. By using scanned illumination, we are able to obtain a high enough seeding density of tracers to measure the full velocity gradient tensor near the rod. The alignment of rods with the vorticity and the eigenvectors of the strain rate show agreement with numerical simulations. A full description of the tumbling of rods in turbulence requires specifying a seven-dimensional joint probability density function (PDF) of five scalars characterizing the velocity gradient tensor and two scalars describing the relative orientation of the rod. If these seven parameters are known, then Jeffery's equation specifies the rod tumbling rate and any statistic of rod rotations can be obtained as a weighted average over the joint PDF. To look for a lower-dimensional projection to simplify the problem, we explore conditional averages of the mean-squared tumbling rate. The conditional dependence of the mean-squared tumbling rate on the magnitude of both the vorticity and the strain rate is strong, as expected, and similar. There is also a strong dependence on the orientation between the rod and the vorticity, since a rod aligned with the vorticity vector tumbles due to strain but not vorticity. When conditioned on the alignment of the rod with the eigenvectors of the strain rate, the largest tumbling rate is obtained when the rod is oriented at a certain angle to the eigenvector that corresponds to the smallest eigenvalue, because this particular orientation maximizes the contribution from both the vorticity and strain.

We introduce a new method to measure Lagrangian vorticity and the rotational dynamics of anisotropic particles in a turbulent fluid flow. We use 3D printing technology to fabricate crosses (two perpendicular rods) and jacks (three mutually perpendicular rods). Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four video images of their motion in a turbulent flow between oscillating grids with R λ = 91. The advected particles have a largest dimension of 6 times the Kolmogorov length, making them a good approximation to anisotropic tracer particles. Crosses rotate like disks and jacks rotate like spheres, so these measurements, combined with previous measurements of tracer rods, allow experimental study of axisymmetric ellipsoids across the full range of aspect ratios. The measured mean square tumbling rate, p ṗi i 〈 〉, confirms previous direct numerical simulations that indicate that disks tumble much more rapidly than rods. Measurements of the alignment of a unit vector defining the orientation of crosses with the direction of their solid-body rotation rate vector provide the first direct observation of the alignment of anisotropic particles by the velocity gradients in a turbulent flow.

The primary goal of this research is to better understand the dynamics of non-spherical particles in turbulence. This includes their preferential alignment with flow structures and their rotations in response to the velocity gradients of the flow or external forces, or both. We perform experimental measurements to study the dynamics of neutrally buoyant fibers and complexshaped particles, and heavy, ramified particles as they sediment under the influence of gravity and turbulence.In 3D homogeneous, isotropic turbulence, we measure the translational and rotational dynamics of small fibers while simultaneously resolving the fluid velocity field around the particles for the first time in experiments. To fully determine the dynamics of fibers in turbulence, it is required to specify a seven-dimensional joint probability density function of five scalars characterizing the velocity gradient tensor and two scalars describing the relative orientation of the fiber. We look at a lower-dimensional projection to simplify the problem and explore conditional averages.The preferential alignment of fibers with the velocity gradient tensor is observed and is in good agreement with direct numerical simulations.The preferential alignment of elongated particles inspired us to design functionalized particles that show a preferential rotation in 3D homogeneous isotropic turbulence. We use 3D printing to fabricate so-called chiral dipoles, a rod with two helices of opposite handedness at either end. High aspect ratio chiral dipoles preferentially align with the extensional stretching field where the helical ends couple to the flow which results in a preferential rotation of the particle.These particles can be used to measure the rate at which fluid elements are stretched, one of the fundamental processes responsible for the energy cascade in turbulent flows.The preferential alignment of non-spherical particles not only depends on particle shape, but also on the density difference between the particles and the fluid. To study the dynamics of heavy, non-spherical particles, we built a new apparatus that allows us to keep particles suspended and independently control the amount of turbulence they experience. We measure orientation distributions of ramified particles, quantify the dependence on turbulence intensity and look at preferential alignment. Moreover, we study the sedimentation and rotation rates and show that under certain conditions, a simple model is sufficient to capture most of the physics. I would like to acknowledge the support of many during my academic career, which comes to an end with this PhD thesis. First and foremost, I would like to thank Professor Greg A. Voth for being a supportive PhD adviser, patient teacher, inspiring scientist and a good friend. I enjoyed every day working with him and in his research group. He guided my professional development and put my interests first. Moreover, he gave me the opportunity to attend numerous national and international conferences, and expand my professional network through ...

In experiments and numerical simulations we measured angles between the symmetry axes of small spheroids advected in turbulence (passive directors). Since turbulent strains tend to align nearby spheroids, one might think that their relative angles are quite small. We show that this intuition fails in general because angles between the symmetry axes of nearby particles are anomalously large. We identify two mechanisms that cause this phenomenon. First, the dynamics evolves to a fractal attractor despite the fact that the fluid velocity is spatially smooth at small scales. Second, this fractal forms steps akin to scar lines observed in the director patterns for random or chaotic two-dimensional maps. arXiv:1707.06037v3 [physics.flu-dyn]

Particles in the shape of chiral dipoles show a preferential rotation in three dimensional homogeneous isotropic turbulence. A chiral dipole consists of a rod with two helices of opposite handedness, one at each end. We can use 3d printing to fabricate these particles with length in the inertial range and track their rotations in a turbulent flow between oscillating grids. High aspect ratio chiral dipoles will align with the extensional eigenvectors of the strain rate tensor and the helical ends will respond to the strain field by spinning around its long axis. The mean of the measured spinning rate is non-zero and reflects the average stretching the particles experience. We use Stokesian dynamics simulations of chiral dipoles in pure strain flow to quantify the dependence of spinning on particle shape. Based on the known response to pure strain, we build a model that gives the spinning rate of small chiral dipoles using Lagrangian velocity gradients from high resolution direct numerical simulations. The statistics of chiral dipole spinning determined with this model show surprisingly good agreement with the measured spinning of much larger chiral dipoles in the experiments.