# Particle-Turbulence Interactions

In this work we examine how particle dynamics and orientation patterns can revel information about fluid structures, how particles can be used as a device to measure fluid statistics, and how in return, fluid structures interact and influence the dynamics of particles. We begin by studying the spatial field of orientations of slender fibers that are advected by a two-dimensional (2D) fluid flow. We introduce emergent scar lines as the dominant coherent structures in the orientation field of passive directors in chaotic flows. We use the standard map as a simple time-periodic two-dimensional flow that produces Lagrangian chaos. This class of flows produces persistent patterns in passive scalar advection, and we find that a different kind of persistent pattern develops in the passive director orientation field. We identify the mechanism by which emergent scar lines grow to dominate these patterns at long times in complex flows. Emergent scar lines form where the recent stretching of the fluid element is perpendicular to earlier stretching. Thus these scar lines can be labeled by their age, defined as the time since their stretching reached a maximum. We next examine how the deformation of particles made of several slender arms in a 2D linear shear and a three-dimensional (3D) turbulent flow allow us to extract the velocity gradient tensor of the flow. Deformation measurements of a particle free to rotate about a fixed axis in a 2D simple shear flow are used to validate our model relating particle deformations to the fluid strain. We then examine deformable particles in a 3D turbulent flow created by a jet array in a vertical water tunnel. Particle positions and orientations are measured with high precision using four high speed cameras and have an uncertainty on the order of $10^{-4}$ radians in particle orientation measurements. Measured deformations in 3D turbulence are small and only slightly larger than our orientation measurement uncertainty. Deformable particles offer a promising method for measuring the full local velocity gradient tensor from measurements of a single particle where traditionally a high concentration of tracer particles would be required. With our capabilities in obtaining particle positions and orientations with high precision we expand upon existing work and study the angular dynamics of large isotropic and anisotropic shaped particles in turbulence and demonstrate capabilities in measuring the various quantities involved in particle dynamics. We examine the shape dependence of particle dynamics in turbulence by looking at large disk-like triads that are made of three slender arms separated by 120 degrees and large spherical tetrads that are made of four slender arms separated by the tetrahedral angle. We see that large particles in turbulence behave differently from the expected tracer spheroids in turbulence result. Furthermore, we take a detailed look into particle alignment with the angular velocity, angular acceleration, and translational acceleration under various conditions such as extreme rotational power events.