This work experimentally examines the geometric characterization and spatial distribution of particle clusters and voids in homogeneous isotropic turbulence. Specifically, we analyze the relationship between turbulent flow structures and the spatial distribution of particles. A Voronoi cell analysis and particle number density are used to identify regions of particle clusters and voids. The turbulent flow structures analyzed are vortices, stagnation points, and fast moving jets.
A 40-cm Eaton box is used to generate a homogeneous isotropic turbulent flow with Reynolds numbers ranging from 157 to 319. The first set of experiments examines the geometric characterization of particle voids and clusters by injecting aluminum-oxide particles with dp = 0.5, 1, 3, and 5μm into four turbulence levels. In the second set of experiments aluminum-oxide particles with dp = 1μm and fluorescent polymer microspheres with dp = 12μm are injected into the Eaton box under four turbulence levels. Images of the aluminum-oxide particles are analyzed using planar Particle Image Velocimetry to identify turbulent flow structures. Simultaneously, a second camera is used to capture images through a filter of the fluorescent polymer microspheres to identify regions of particle clusters and voids. The spatial distribution of voids and clusters are compared with the distribution of turbulent flow structures.
The characteristic length scales for particle voids and clusters vary significantly between the two sets of experiments. The images of polymer microspheres, which have a greater resolution, indicate a characteristic length scale for particle clusters of 10-20η depending on the Reynolds number, which matches the results of previous studies. Similarly, we find the characteristic length scales of particle voids ranges from 10 to 100η. In both sets of experiments, we find the distribution of particle voids, and to some extent particle clusters, follow a power-law fit indicating self-similarity. Results are inconclusive in linking turbulent flow structures to particle clusters and voids, which was unexpected.
