Experimental investigation of inertial sphere, rod, and disk particles in a turbulent boundary layer
2021-06
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Experimental investigation of inertial sphere, rod, and disk particles in a turbulent boundary layer
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2021-06
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Turbulent, particle-laden flows are ubiquitous in nature and industry. Particles in many of these flows have finite size and inertia, which cause them to interact with the fluid turbulence in complex ways. They are also commonly non-spherical in shape, which adds further richness to the particle-fluid interplay. In this thesis, the dynamics of dilute, slightly negatively buoyant particles, fully suspended in a smooth-wall open channel flow, are investigated experimentally. Spheres, disks, and rods are studied in order to examine the effects of particle shape on their distribution and interaction with the fluid turbulence. The friction Reynolds number of the flow is $Re_\tau \sim 600$, and the particle Stokes number based on the friction velocity is $St^+ \sim O(10)$. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) are used to obtain simultaneous, time-resolved flow fields and particle trajectories. Their translational and rotational motion, as well as their concentration and dispersion, are investigated. Disks and rods are both found to oversample high-speed fluid near the wall, in agreement with particle-resolved DNS studies. The spherical particle Reynolds stresses exceed those of the fluid due to particle trajectories crossing fluid streamlines; this effect is not observed for rods and disks. Spherical particle transport is strongly linked to ejections, while the role of sweeps is marginal, and there is no evidence of turbophoresis. The mean concentration profile of the spheres follows a power-law with a shallower slope than predicted by equilibrium theories that neglect particle inertia. However, rod and disk mean concentration profiles follow Rouse-Prandtl theory over a large portion of the boundary layer. Particle diffusivity is shown to be well-approximated by the fluid eddy diffusivity. A detailed investigation of sphere behavior near the wall is carried out. Upward-/downward-moving particles display positive/negative mean streamwise acceleration due to the particle--fluid slip. The particles that contact the wall are faster than the local fluid both before reaching the wall and after leaving it. Therefore, they are decelerated by drag and pushed downward by shear-induced lift. The durations of wall contact follow exponential distributions with characteristic timescale close to the particle response time. Lift-offs coincide with particles meeting a fluid ejection. These observations emphasize the competing effects of inertia and gravity. The orientation and rotation of rod and disk particles are also measured. Rods tend to orient mostly in the streamwise direction, while disks strongly prefer to align their symmetry axis mostly normal to the wall. This alignment is much more stable for disks than for rods. Rods undergo strong tumbling near the wall and tend to tumble freely in response to the mean shear and turbulent fluid velocity fluctuations, whereas disks tend to wobble about their preferential wall-normal orientation, resulting in much weaker tumbling rates close to the wall. Wall contact is also implicated as a significant tumbling-inducing mechanism. Many of these results have not been previously confirmed experimentally.
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University of Minnesota Ph.D. dissertation. June 2021. Major: Aerospace Engineering and Mechanics. Advisor: Filippo Coletti. 1 computer file (PDF); ix, 96 pages.
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Baker, Lucia. (2021). Experimental investigation of inertial sphere, rod, and disk particles in a turbulent boundary layer. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/224538.
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