The radial relative velocity (RV) between particles suspended in turbulent flow plays a critical role in droplet collision and growth. We present a simple and accurate approach to RV measurement in isotropic turbulence—planar 4-frame particle tracking velocimetry—using routine PIV hardware. It improves particle positioning and pairing accuracy over the 2-frame holographic approach by de Jong et al. (Int J Multiphas Flow 36:324–332; de Jong et al., Int J Multiphas Flow 36:324–332, 2010) without using high-speed cameras and lasers as in Saw et al. (Phys Fluids 26:111702, 2014). Homogeneous and isotropic turbulent flow (Rλ=357\documentclass[12pt]{minimal}
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\begin{document}$${R_\lambda }=357$$\end{document}) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean St=0.09\documentclass[12pt]{minimal}
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\begin{document}$$St=0.09$$\end{document}, standard deviation 0.05) or high-Stokes aerosols (mean St=3.46\documentclass[12pt]{minimal}
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\begin{document}$$St=3.46$$\end{document}, standard deviation 0.57). For comparison, DNS was conducted under similar conditions (Rλ=398\documentclass[12pt]{minimal}
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\begin{document}$${R_\lambda }=398$$\end{document}; St=0.10\documentclass[12pt]{minimal}
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\begin{document}$$St=0.10$$\end{document} and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with St\documentclass[12pt]{minimal}
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\begin{document}$$St$$\end{document} at small particle separations, r\documentclass[12pt]{minimal}
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\begin{document}$$r$$\end{document}, and decreases with St\documentclass[12pt]{minimal}
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\begin{document}$$St$$\end{document} at large r\documentclass[12pt]{minimal}
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\begin{document}$$r$$\end{document}, indicating the dominance of “path-history” and “inertial filtering” effects, respectively. However, at small r\documentclass[12pt]{minimal}
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\begin{document}$$r$$\end{document}, the experimental mean inward RV trends higher than DNS, possibly due to the slight polydispersity of particles and finite light sheet thickness in experiments. To confirm this interpretation, we performed numerical experiments and found that particle polydispersity increases mean inward RV at small r\documentclass[12pt]{minimal}
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\begin{document}$$r$$\end{document}, while finite laser thickness also overestimates mean inward RV at small r\documentclass[12pt]{minimal}
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\begin{document}$$r$$\end{document}, This study demonstrates the feasibility of accurately measuring RV using routine hardware, and verifies, for the first time, the path-history and inertial filtering effects on particle-pair RV at large particle separations experimentally.
