Effects of Flow Structures in Homogeneous Isotropic Turbulence on Particle Clustering

In clouds, the main growth mechanism of droplets with diameters 10-50 µm, known as the size-gap, is collision and coalescence. Atmospheric turbulence is known to increase the droplet growth rate in this range by enhancing the relative velocity between droplets and the formation of droplet clustering, thus, increasing the droplet collision rate. The focus here is to further understand droplet clustering by experimentally exploring the relationship between homogeneous isotropic turbulent flow structures and particle clustering. A 40-cm Eaton box is used to generate a homogeneous turbulent flow in which aluminum-oxide and fluorescent polymer particles are injected. Images of the aluminum-oxide particles with diameters of 0.5 µm are taken and analyzed using Particle Image Velocimetry to determine the flow structures present. Simultaneously, a second camera is used to capture images through a filter of the fluorescent polymer particles with diameters of 15 µm. Various optical tools are used to overlap the region of interest in the two sets of images. The relationship between particle concentration and flow structures is examined for a range of flow conditions, where clustering is quantified using a Voronoi cell analysis and the particle number density.