Particle-pair relative velocity measurement in high-Reynolds-number homogeneous and isotropic turbulence using 4-frame particle tracking velocimetry

• Published in: Experiments in fluids 2018-02, Vol.59 (2), p.1-17, Article 30
• Main Authors: Dou, Zhongwang, Ireland, Peter J., Bragg, Andrew D., Liang, Zach, Collins, Lance R., Meng, Hui
• Format: Article
• Language: English
• Subjects: Aerodynamics; Computational fluid dynamics; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Feasibility studies; Filtration; Fluid flow; Fluid- and Aerodynamics; Hardware; Heat and Mass Transfer; Isotropic turbulence; Particle tracking; Particle tracking velocimetry; Polydispersity; Probability density functions; Research Article; Standard deviation; Stokes law (fluid mechanics); Turbulence; Turbulent flow; Velocity; Velocity measurement
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-017-2481-0

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 λ = 357 ) in a new, fan-driven, truncated iscosahedron chamber was laden with either low-Stokes (mean S t = 0.09 , standard deviation 0.05) or high-Stokes aerosols (mean S t = 3.46 , standard deviation 0.57). For comparison, DNS was conducted under similar conditions ( R λ = 398 ; S t = 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 S t at small particle separations, r , and decreases with S t 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 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 , 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.