A simple experiment for observing clustering and dynamics of coalescing particles in air turbulence

Abstract

A novel experimental platform is developed to investigate the dynamics of inertial particles (micro-droplets) in air turbulence. The goal is to observe particle collision and coalescence in turbulent flows, focusing on its impact on the radial distribution function (RDF) and relative velocity statistics. The main tool is a three-dimensional Lagrangian particle tracking (LPT) system, designed for high-resolution measurements at sub-Kolmogorov scales. The system uses LED illumination with high-speed spinning-disk atomizers, enabling tracking of particles of approximately 10~$μ$m and larger under controlled turbulence. A minimum resolvable particle separation of $r/η\approx 0.1$ is achieved. A central contribution is the identification and mitigation of three dominant sources of spurious particles: FMIS, IIS, and TIF. An angle-based geometric filtering criterion strongly suppresses FMIS artifacts on RDF. These procedures establish a validated workflow for reliable small-scale statistics. Using this framework, RDF and a normalized pseudo-collision rate are measured at near-contact separations for particles with Stokes numbers $St \approx 0.2$--$1.0$. Sub-Kolmogorov clustering increases with Stokes number, and near-contact statistics are consistent through the filtering strategy. This study extends LPT limits and provides a reliable methodology for investigating inertial-particle dynamics at previously inaccessible spatial scales.

Figures (16)

• Figure 1: Schematic and picture of the LPT experimental system. Panel (a) illustrates the optical arrangement, including the octagonal turbulence chamber (height 300 mm, flat-to-flat distance 250 mm). Three high-speed cameras are arranged around the chamber at approximately 45° intervals for stereoscopic particle tracking. Illumination is provided by high-power LED arrays positioned opposite to each camera and aligned with the optical axis. Panel (b) shows the three cameras mounted on adjustable mechanical stages for precise alignment, while the turbulence chamber is fixed between two optical tables.
• Figure 2: Particle generation system based on spinning disk. Panel (a) two counter-rotating disks (diameter $4\,\mathrm{cm}$) are positioned at the upper and lower boundaries of the turbulence chamber. Liquid is supplied to both and is atomized into fine droplets by the strong centrifugal force produced by high-speed rotation. The droplets are subsequently mixed into the turbulence chamber. Panel (b) the disks are mounted coaxially and driven by high-speed motors. Droplet sizes are controlled by the speed of the spinning disks.
• Figure 3: Particle size distribution. The x-axis represents the particle diameter ($\mu\mathrm{m}$), while the y-axis shows the probability density function (PDF) of the particle size. The red circular curve corresponds to 42,000 rpm, the blue rectangular curve to 30,000 rpm, and the brown star-shaped curve to 18,000 rpm.
• Figure 4: Single-particle trajectory. The X-, Y-, and Z-axes denote the horizontal, vertical (gravity-aligned), and imaging-depth directions, respectively. The black arrow indicates the particle’s direction of motion.
• Figure 5: Particle Tracks in 3D Space during a Short Time Interval (0.0034s). Each curve represents the trajectory of a single particle moving in three-dimensional space over time, with different colors and marker styles used to distinguish individual droplets. No tracks from different times have been combined; all trajectories are measured simultaneously. The black arrow indicates the direction of movement of the droplets.