Publication Date

5-2018

Advisor(s)

Greg A. Voth

Department

Physics

Abstract

The primary goal of this research is to better understand the dynamics of non-spherical particles in turbulence. This includes their preferential alignment with ow structures and their rotations in response to the velocity gradients of the ow or external forces, or both. We perform experimental measurements to study the dynamics of neutrally buoyant fibers and complex-shaped 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 ow 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.

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