Measurements of the radial distribution function of inertial particles in turbulent flows.

Inertial~particles (particles with a higher density than that of the surrounding fluid) in turbulent flows cluster: they are ejected from regions of high vorticity and concentrate in regions of high strain. The Radial Distribution Function (RDF) [S. Sundaram and L.R. Collins, J. Fluid Mech. 335, 1997; R. A.~Shaw et al, Q. J.R. Met. Soc. 128, 2002]~is used to quantify clustering. Computations show that it peaks for particle separations in the dissipation range. Here we describe~measurements of~two-dimensional RDFs in decaying high Reynolds number wind tunnel turbulence. The turbulence is produced using an active grid and the inertial particles are generated by water sprays. For this poly-dispersed droplet distribution the size distribution is determined using a phase doppler particle analyzer. The particles are illuminated using a laser~light~sheet and their positions are obtained using high speed cameras downstream of the grid. Our results are compared with measurements using a~phase doppler interferometer~in the same flow, and with other recent measurements in box turbulence.