Inertial Particles And Entrainment In Turbulent Flows
This work consists of three independent experimental studies. The first studies inhomogeneous turbulence with (even symmetric) non-Gaussian velocity probability density functions, created by partitioning an active grid in a wind tunnel. The turbulence is like that encountered in the environment (e.g., by wind turbines) or produced by fractal grids. The statistics of the turbulence are shown in particular to depend simply upon the spatial derivatives of the velocity r.m.s. field. The second study characterizes the intermittent nature of an inertial particle mixing layer, where a particle-laden flow entrains a particle-free flow. This is the case at the boundary of a cloud and affects its growth and hence climate modelling. The flow is accomplished with water droplets in a wind tunnel downstream of a splitter plate; the experiments are done for both homogeneous turbulence and where the active grid is partitioned to create an inhomogeneous interface like in the first study. We show the droplets to be entrained in intermittent, large-scale bursts, preserving the properties of the air and turbulence ambient to them, which in a cloud are critical to droplet growth. Gravitational settling effects are isolated by rotation of the apparatus. The third and final study takes a more fundamental look at inertial particles in isotropic conditions using both numerical simulations and water droplets in air turbulence from 32 loudspeaker jets aimed at a central point. Turbulent settling speeds, velocity variance and variance anisotropy are measured over an unprecedented parameter range, identifying all relevant parameterizations of particle inertia and buoyancy affecting their response to turbulence, with applications to all particle-carrying flows. The three mechanisms by which turbulence modifies settling speeds (as compared to in quiescent flow) are isolated for the first time, with vertical turbulent motions shown to be responsible for settling reductions and horizontal ones to be for enhancements. The three experiments rely on hot-wire anemometry, phase Doppler particle analysis (PDPA) and high-speed camera Lagrangian particle tracking techniques. The body of the text consists of three stand-alone papers. The dissertation work provides fundamental understanding for wide-ranging environmental and industrial engineering problems.
Inertial particles; entrainment; turbulence
Cowen III, Edwin Alfred; Collins, Lance; Bodenschatz, Eberhard
Ph.D. of Mechanical Engineering
Doctor of Philosophy
dissertation or thesis