Motion of inertial particles with size larger than Kolmogorov scale in turbulent flows

We report experimental results on the motion of tracer and non-tracer particles in intense turbulent water flows between counter-rotating disks measured by three-dimensional Lagrangian particle tracking. The sizes of the non-tracer particles were in the range of eta < d(p) << L, where eta is the Kolmogorov length scale and L is the integral scale. We propose a modified Stokes number that takes into account the effects from finite particle size and inertia. We compare results from tracers and from two types of particles (heavy+small, approx. neutrally bouyant+large) for which the conventional Stokes numbers differ by a factor of approximate to 8% and the modified Stokes numbers by approximate to 60%. The conventional Stokes numbers of the particles investigated were in the range of 0.7 and 1.5, while the modified Stokes numbers were smaller between 0.1 to 0.3. We observed that the tails of the measured acceleration PDFs were slightly narrower compared to tracer particles with the heavier+smaller particles showing a larger effect. The measured Lagrangian acceleration correlations of the large particles were approximately the same as that of the tracer particles. This suggests that trajectories of large particles are not biased towards the low-vorticity, high-straining region as was observed previously in the case of small, very heavy particles. These findings are also supported by the measurements of the local slopes of the fourth order Lagrangian structure functions. (c) 2008 Elsevier B.V. All rights reserved.