Non-Spherical Particle Dynamics in Turbulence

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 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 ...

Importance of fluid inertia for the orientation of spheroids settling in turbulent flow

“…A theory for the orientation distribution for , when the fluid-inertia torque dominates, has been derived by Gustavsson et al. (2019); see also Kramel (2017).…”

“…In this case the fluid-inertia torque dominates the angular dynamics, leading to a distribution of orientation maximising the drag, at odds with the predictions of models neglecting this torque (Siewert et al 2014a,b;Gustavsson et al 2017;Jucha et al 2018;Naso et al 2018). A theory for the orientation distribution for R > 1, when the fluid-inertia torque dominates, has been derived by Gustavsson et al (2019); see also Kramel (2017). Tables 1 and 2 list the parameters used in the simulations of figure 3 and 4, respectively.…”

“…The second term on the r.h.s. of (2.4) represents the torque induced by the non-Galilean nature of the particle frame (Landau & Lifschitz 1976). The first term on the r.h.s.…”

“…Lopez & Guazzelli (2017) also performed numerical simulations of a model taking into account convective fluid inertia, generally Orientation of spheroids settling in turbulent flow 886 A9-3 in good agreement with their experimental results. Kramel (2017) analysed the angular dynamics of slender rods settling in turbulence at various Reynolds numbers, and showed that rods settle with their broad side first. The observed distribution of orientation is very narrow, the more so as the settling number Sv (a dimensionless measure of the settling velocity) is large.…”

Lord Kelvin's Error? An Investigation into the Isotropic Helicoid

“…Since the analytic expressions for the rotational translation coupling tensors becomes cumbersome, we numerically evaluate the expressions in MatLab. We use as 3/2 for ka p since that is the value for a disc in the creeping flow limit [14]. Later, we will show that the rotational translational coupling tensor is identically zero, which means that the result is independent of the value of ka p .…”