Orientation dynamics of oddly-shaped particles
Mees Flapper is a PhD student in the department Physics of Fluids. (Co)Promotors are prof.dr. D. Lohse and dr. S.G. Huisman from the faculty of Science & Technology, University of Twente.
Throughout natural phenomena and industrial processes, studying particle-laden flows is vital for predicting and optimising particle-fluid systems. This dissertation investigates oddly-shaped particles, where we purposely deviate from spherical particles, as spherically symmetric particles are the exception in natural and industrial particle-laden flows. The dynamics of oddly-shaped particles is studied for various particle shapes and numerous flows, investigating the effect of particle shape and turbulence.
Chapter 1 describes an algorithm for tracking the location and orientation of anisotropic particles. This method uses the particle silhouette recorded by multiple cameras to determine the particle orientation. By generating a synthetic (computer-generated) particle with a known orientation, we compare the silhouette of the synthetic particle and the experimentally recorded particle. The silhouettes of the experimental and synthetic particle are compared: the difference between the silhouettes is minimised by varying the synthetic particle's orientation. The resulting orientation of the synthetic particle then gives the orientation of the tracked particle.
The orientation algorithm is shown to be accurate and robust, with the orientation error generally being smaller than 0.1° when using multiple cameras. Various particle shapes are tracked using the algorithm, which are all reliably and accurately tracked. The effects of image noise, image size, number of cameras, and relative camera orientations are all explored, showing how these factors affect the orientation error.
Chapter 2 uses the orientation tracking algorithm developed in chapter 1. The settling of chiral particles (which break mirror symmetry) is studied by tracking the particle orientation over time. The 3D-printed particles are heavier than water, and settle in water in a Dodecahedral tank. The settling dynamics of the particles are studied in quiescent water, and various turbulence levels, ranging from . In the quiescent case, the particles often settle in a stable manner, where the particles rotate around the vertical axis. Here the rotation direction is determined by the particle chirality, exemplifying chiral translation-rotation coupling. Besides the stable settling, the particles are observed to tumble (where the rotation and orientation are randomly distributed), and more rarely, the particles are observed to descend as a `corkscrew'.
Introducing turbulence in the settling dynamics is done by rotating 20 propellers in the Dodecahedral tank, where the turbulence intensity is varied by the propeller speed. Increasing the turbulence leads to larger horizontal motions of the particle, and decreases the mean (vertical) settling velocity. Additionally, the stable settling mode is observed less frequently, and the tumbling mode more frequently as the turbulence intensity increases. This indicates the decreasing importance of particle geometry and chirality as the turbulence intensity is increased.
Chapter 3 extends on the findings of chapter 2, now studying chiral particles in a Taylor--Couette flow. By counterrotating the two concentric cylinders with a factor between the rotations of the inner and outer cylinder, the flow develops strong Taylor vortices, with a mean flow in the azimuthal direction (around the inner cylinder). These vortices are present throughout the setup, being stacked vertically. The rotation direction of the stacked vortices (the `spiralling' direction) alternate axially.
Employing the preferential rotation direction of the chiral particles found in chapter 2, the chiral particles are studied in the described Taylor--Couette flow, investigating whether any difference between the particle chiralities are present. Over a range of high Reynolds numbers (), the chiral particles are tracked in the Taylor--Couette flow. From the results, no difference in dynamics between the particle chiralities is found, and do not display any preferential clustering (left-handed particles in left-handed vortices) or rotation. Similarly, the orientation of the chiral particles in Taylor--Couette flow is found to be randomly distributed, as opposed to the preferential alignment as seen at low Reynolds number in chapter 2. Instead, the particles' dynamics is flow-dominated, as shown by the particle rotation being determined by the vortex rotation direction.
Chapter 4 investigates a different particle shape, where the settling dynamics of an oloid is studied. This shape is anisotropic but is mirror symmetric. The settling dynamics of an oloid in a quiescent fluid are studied using experiments and simulations, which show strong agreements in results. Two main settling regimes are identified: a stable settling mode, in which the oloid preferentially rotates around the vertical axis, and a tumbling mode, where the rotation and orientation of the oloid are randomly distributed. In the stable settling mode, the rotation direction is determined by the oloid's initial orientation.
The fact that the stable oloid rotates around the vertical axis is striking, since the mirror-symmetric shape of the oloid does not suggest a translation-rotation coupling a priori. Numerical results indicate that the oloid's initial orientation determines the rotation direction. The stable settling oloid occurs in the Stokes regime, whereas the oloid tumbles outside the Stokes regime. The experiments and simulations both provide evidence for a transition between the aforementioned falling modes.