Experimental study on clustering of large particles in homogeneous turbulent flow

Results of simultaneous measurements of fluid flow and motion of large solid particles in homogeneous turbulent flow (at the Taylor microscale Reynolds number Re_λ = 250) are presented. Velocity, acceleration and spatial distribution of particles in a three-dimensional volume along with the surrounding flow velocity and velocity derivatives fields (incl. vorticity and the strain-rate tensor) are obtained by means of two synchronized and cross-calibrated three-dimensional particle tracking velocimetry (3D-PTV) systems. The main focus of the present investigation is on the two-way coupling between the turbulent flow and solid particles which are significantly larger (∼900 μm) than the Kolmogorov length scale (∼200 μm) and heavier than the surrounding liquid (ρ_p = 1400 kg m^-3 > ρ_f = 1000 kg m^-3). Joint statistics of the local particle concentration and the local strain and vorticity fields indicate that (1) large particles tend to cluster in strain-dominated regions and that (2) preferential concentration (clusters and voids) occur on scales comparable with the Taylor microscale, λ. We infer that the observed clustering of large particles can be related to the same mechanisms applying for clustering of sub-Kolmogorov size particles and that, in analogy, the time scale λ/u_r.m.s. can replace the Kolmogorov time scale as a normalization of the particle response time in the definition of the Stokes number.