We present an experimental study of the preferential concentration of sub-Kolmogorov inertial particles in active-grid-generated homogeneous and isotropic turbulence, characterized via Voronoi tessellations. We show that the detection and quantification of clusters and voids are influenced by the intensity of the laser and high values of particles volume fraction ${\varphi }_v$. Different biases on the statistics of Voronoi cells are analyzed to improve the reliability of the detection and the robustness in the characterization of clusters and voids. We do this by adapting big-data techniques that allow us to process the particle images up to 10 times faster than standard algorithms. Finally, as preferential concentration is known to depend on multiple parameters, we perform experiments where one parameter is varied and all others are kept constant (${\varphi }_v$, the Reynolds number based on the Taylor length scale ${\text{Re}}_{\lambda }$, and residence time of the particles interacting with the turbulence). Our results confirm, in agreement with published work, that clustering increases with both ${\varphi }_v$ and ${\text{Re}}_{\lambda }$. On the other hand, we find evidence that the mean size of clusters increases with ${\text{Re}}_{\lambda }$ but decreases with ${\varphi }_v$ and that the cluster settling velocity is strongly affected by ${\text{Re}}_{\lambda }$ up to the maximum value studied here, ${\text{Re}}_{\lambda }=250$.

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• Particle-laden flows
• Turbulence