Experimental study of aerosol clustering in isotropic turbulence by holographic imaging
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A ground-based experimental study of particle clustering is conducted in isotropic turbulence generated in laboratory by holographic particle imaging technique. The particle radial distribution function (RDF) has been identified as a key variable for quantifying the effect of clustering on binary processes such as collision. Measurements of the RDF are best done in three dimensions (Holtzer & Collins 2002); hence most results to date come from direct numerical simulations at modest Reynolds numbers without experimental validation. To complement DNS database and valid theory predictions of particle clustering, we apply the film-based analogy holographic particle image velocimetry (HPIV) and the digital HPIV to perform three-dimensional measurements of particle clustering in nearly homogeneous isotropic turbulence in a box. The turbulence corresponding to different fan speeds has been characterized using particle image velocimetry (PIV), and the turbulent energy dissipation rate was obtained from a fit of the second-order structure function. Metal-coated hollow glass spheres with a well defined particle size distribution were injected into the turbulent flow. The three-dimensional snapshots of particles distribution within the test volume were obtained and computed for RDF profiles. The film-based HPIV system recorded the first 3D particle clustering result up to date, and confirmed the attenuation in RDF inherent from lower dimension photography methods, which inspire later more advanced 3D measurements by digital HPIV system. From those, the three-dimensional RDF was observed to increase with time from injection. By phase averaging the measurements based on the time from injection, it was possible to semi-quantitatively measure the temporal evolution of the RDF. This is believed to be an important feature of the RDF in practical industrial applications (e.g., powder manufacturing) and naturally occurring flows (e.g., cloud droplets), where temporal dynamics may result from changes in the local conditions and/or droplet coalescence (Reade & Collins 2000). There are good agreements of the RDF profiles between the steady state of experimental results and those from simulations when particle polydisperse size distribution is considered. This is the first agreement between experiments and simulations at parametric overlap. The disagreements also point out the potential uncertainties in particle size and clustering phases, which inspire experiments at more well controlled conditions. The digital HPIV system is also calibrated and optimized for the extended investigations on Reynolds number dependence study which has more wide applications for nature flows.