Preferential concentration of poly-dispersed droplets in stationary isotropic turbulence

The preferential concentration of poly-dispersed water droplets with a range of Sauter mean diameters between 25 and 95 μm has been studied experimentally in stationary homogeneous isotropic turbulence with four different intensities, characterized by turbulent Reynolds numbers based on Taylor microscale, of Re _λ  = 107, 145, 185 and 213. The image processing method of recorded scattered light intensity images from droplets is described and its ability to identify droplets is assessed in terms of image quality. The influence of image processing parameters on measured characteristics of droplet clustering is evaluated. The radial distribution function (RDF) and 2D Voronoï analysis quantified the magnitude of preferential droplet concentration and the results from both methods agreed well. RDF showed that the characteristic length scale of resulting droplet clusters varies between 20 and 30 times the Kolmogorov length scale over all the experimental conditions. It was found that the preferential concentration is more appropriately described by a Stokes number, based on various representative diameters, namely the arithmetic mean diameter, D ₁₀, or the diameter, DN_60 %, below which 60 % of the total droplet number in the spray is present, or the diameter, DV_5 %, which carries 5 % of the total liquid volume in the spray. The magnitude of droplet preferential concentration was maximum when the proposed Stokes number was around unity for all experimental conditions. Little dependence of the magnitude of preferential concentration on turbulent Reynolds numbers was found, in contrast to the recent DNS findings (Tagawa et al. in J Fluid Mech 693:201–215, 2012).