PhD Dissertation Defense: Si Chen

The PhD Dissertation Defense of Si Chen is presented by the University of North Carolina at Chapel Hill’s, Department of Marine Sciences. This event will be held on Tuesday, July 7th at 3:00 pm. This event will be held ONLINE ONLY and it will be available online via Zoom (Use Zoom Meeting ID: 925 1214 5064).

Title: Particle Collision in Grid Turbulence

Abstract: Collisions between particles suspended in a fluid play an important role in many natural and industrial processes. Although both theoretical and numerical investigations have been conducted extensively in understanding the particle collisions in different fluid, flow conditions and particle properties, direct measurement of particle collision is very challenging and limited. In this dissertation, particle image velocimetry and particle tracking velocimetry are used to experimentally study zero-inertia solid particle collisions in both homogeneous isotropic turbulence and two-layer stably stratified turbulence in laboratory experiments.
Two-dimensional particle image velocimetry experiments are first conducted to calculate the characteristics of oscillating grid turbulence. The turbulence is found isotropic and homogeneous in the area of interest. The turbulent dissipation rate under three different flow conditions are calculated by both direct method and structure function method which provide comparable results. Second, both kinematic and dynamic collision kernels of solid zero-inertia particles in isotropic turbulence at low- to intermediate-Reynolds number with real particle diameters smaller than the Kolmogorov length scales are measured experimentally. Both kinematic and dynamic collision kernels are compared with Saffman and Turner (1956) theory. The deviation at small distance is identified by the effect of hydrodynamic interactions while at larger distance, finite-size effects and lubrication effects become the main contributors. The present method allows for direct measurements, excluding multiple collisions from real (initial) collision for the calculation of the dynamic collision kernels. Last, kinematic collision kernels of solid zero-inertia particles in two-layer stratified turbulence are measured. The two major drivers of the collision kernels, i.e. the particle relative velocity and the particle preferential concentration, are playing opposite roles in the stratified turbulence. On the one hand, the stratified environment confines the floating particles near the density interface and thus increases the particle concentration. On the other hand, the motions of the particles confined within this region are also suppressed by the stratification and thus have lower relative velocity. The net effect of preferential concentration and turbulent transport is that the former outperforms the latter, leading to a higher collision kernel in the stratified system than the homogeneous system considered in the present study.