Effects of Reynolds number and Stokes number on particle-pair relative velocity in isotropic turbulence: a systematic experimental study

摘要

The effects of Reynolds number (R-lambda) and Stokes number (St) on particle-pair relative velocity (RV) are investigated systematically using a recently developed planar four-frame particle tracking technique in a novel homogeneous and isotropic turbulence chamber. We compare the measured results with direct numerical simulation (DNS), verifying whether the conclusions of the DNS for simplified conditions and limited R-lambda are still valid in reality. Two experiments are performed: varying R-lambda between 246 and 357 at six St values, and varying St between 0.02 and 4.63 at five R-lambda values. The measured mean inward particle-pair RV w(r)(-) as a function of separation distance r is compared with the DNS under closely matched conditions. At all experimental conditions, an excellent agreement is achieved, except when the particle separation distance r less than or similar to 10 eta (eta is the Kolmogorov length scale), where the experimental w(r)(-) is consistently higher, possibly due to particle polydispersity and finite laser thickness in the experiments (Dou et al., arXiv:1712.07506, 2017). At any fixed St, (w(r)(-)) is essentially independent of R-lambda, echoing the DNS finding of Ireland et al. (J. Fluid Mech., vol. 796, 2016, pp. 617-658). At any fixed R-lambda, W-r(-) increases with St at small r, showing dominance of the path-history effect in the dissipation range when St less than or similar to 0(1), but decreases with St at large r, indicating dominance of inertial filtering. We further compare the w(r)(-) and RV variance w(r)(2) from experiments with DNS and theoretical predictions by Pan & Padoan (J. Fluid Mech., vol. 661, 2010, pp. 73-107). For St less than or similar to 1, experimental w(r)(-) and (w(r)(2)) match these values well at r greater than or similar to 10 eta, but they are higher than both DNS and theory at r less than or similar to 10 eta. For St greater than or similar to 1, (w(