Article: Dou, ZW; Ireland, PJ; Bragg, AD; Liang, Z; Collins, LR; Meng, H; “Particle-pair Relative Velocity Measurement in High-Reynolds-number Homogeneous and Isotropic Turbulence using 4-frame Particle Tracking Velocimetry”, Experiments in Fluids, 59 (2)
Abstract: 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 (R-lambda = 357) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean St = 0.09, standard deviation
0.05) or high-Stokes aerosols (mean St = 3.46, standard deviation 0.57).
For comparison, DNS was conducted under similar conditions (R-lambda = 398; St = 0.10 and 3.00, respectively). Experimental RV probability density functions (PDF) and mean inward RV agree well with DNS. Mean inward RV increases with St at small particle separations, r, and decreases with St at large r, indicating the dominance of “path-history” and “inertial filtering” effects, respectively. However, at small r, the experimental mean inward RV trends higher than DNS, possibly due to the slight poly-dispersity 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, while finite laser thickness also overestimates mean inward RV at small r, 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.
Funding Acknowledgement:
National Science Foundation [CBET-0967407, CBET-0967349, ACOR0001, P35091057, ark:/85065/d7wd3xhc]; National Science Foundation
Funding Text:
This work was supported by the National Science Foundation through Collaborative Research Grants CBET-0967407 (HM) and CBET-0967349 (LRC) and through a graduate research fellowship awarded to PJI. We would also like to acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory through Grants ACOR0001 and P35091057, sponsored by the National Science Foundation. We thank Adam L. Hammond for highly valuable technical and editorial assistance. We also thank Dr. Lujie Cao in Ocean University of China for initiating and helping in the implementation of the planar 4-frame PTV technique.