  Separate and joint droplets, clusters, and voids characteristics of sprays
injected in a turbulent co-flow are investigated experimentally. Simultaneous
Mie scattering and Interferometric Laser Imaging for Droplet Sizing along with
separate hotwire anemometry are performed. The turbulent co-flow
characteristics are adjusted using zero, one, or two perforated plates. The
Taylor length scale-based Reynolds number varies from 10 to 38, and the Stokes
number estimated based on the Kolmogorov time scale varies from 3 to 25. The
results show that the mean length scale of the clusters normalized by the
Kolmogorov length scale varies linearly with the Stokes number. However, the
mean void length scale is on the order of the integral length scale. It is
shown that the number density of the droplets inside the clusters is about 10
times larger than that in the voids. The ratios of the droplets number
densities in the clusters and voids to the total number density are independent
of the test conditions and equal 8.3 and 0.7, respectively. The joint
probability density function of the droplets diameter and clusters area shows
that the droplets with the most probable diameter are found in the majority of
the clusters. It is argued that intensifying the turbulence broadens the range
of turbulent eddy size in the co-flow which allows for accommodating droplets
with a broad range of diameters in the clusters. The results are of
significance for engineering applications that aim to modify the clustering
characteristics of large Stokes number droplets sprayed into turbulent
co-flows.
