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In this study, we analyze the statistics of both individual inertialparticles and inertial particle pairs in direct numerical simulations ofhomogeneous isotropic turbulence in the absence of gravity. The effect of theTaylor microscale Reynolds number $R_\lambda$ on the particle statistics isexamined over the largest range to date (from $R_\lambda = 88-597$). We firstexplore the effect of preferential sampling on the single-particle statistics,and use our understanding of preferential sampling to provide a physicalexplanation for many of the trends in the particle velocity gradients, kineticenergies, and accelerations at low $St$. As $St$ increases, inertial filteringeffects become more important, causing the particle kinetic energies andaccelerations to decrease.  We then consider particle-pair statistics, and focus our attention on therelative velocities and radial distribution functions (RDFs) of the particles.The relative velocity statistics indicate that preferential-sampling effectsare important for $St \lesssim 0.1$ and that path-history/non-local effectsbecome increasingly important for $St \gtrsim 0.2$. The lower-order relativevelocity statistics are only weakly sensitive to changes in Reynolds number atlow $St$. We find that the RDFs peak near $St$ of order unity, that theyexhibit power-law scaling for low and intermediate $St$, and that they arelargely independent of Reynolds number for low and intermediate $St$. We alsoobserve that at large $St$, changes in the RDF are related to changes thescaling exponents of the relative velocity variances. The particle collisionkernel is found to be largely insensitive to the flow Reynolds number,suggesting that relatively low-Reynolds-number simulations may be able tocapture much of the relevant physics of droplet collisions and growth in theadiabatic cores of atmospheric clouds.

The effect of Reynolds number on inertial particle dynamics in isotropic turbulence. Part I: Simulations without gravitational effects