J. Fluid Mech. (2002), vol. 468, pp. 77–105. c  2002 Cambridge University Press DOI: 10.1017/S0022112002001593 Printed in the United Kingdom 77 Effect of preferential concentration on the settling velocity of heavy particles in homogeneous isotropic turbulence By A. ALISEDA, A. CARTELLIER †, F. HAINAUX AND J. C. LASHERAS Department of Mechanical and Aerospace Engineering, University of California San Diego, La Jolla, CA 92093, USA (Received 26 July 2001 and in revised form 16 April 2002) The behaviour of heavy particles in isotropic, homogeneous, decaying turbulence has been experimentally studied. The settling velocity of the particles has been found to be much larger than in a quiescent fluid. It has been determined that the enhancement of the settling velocity depends on the particle loading, increasing as the volume fraction of particles in the flow increases. The spatial and temporal distribution of the particle concentration field is shown to exhibit large inhomogeneities. As the particles interact with the underlying turbulence they concentrate preferentially in certain regions of the flow. A characteristic dimension of these particle clusters is found to be related to the viscous scales of the flow. Measurements of the settling velocity conditioned on the local concentration of particles in the flow have shown that there is a monotonic increase in the settling velocity with the local concentration (the relation being quasi-linear). A simple phenomenological model is proposed to explain this behaviour. 1. Introduction The evolution of particle-laden flows is relevant to many industrial and environ- mental processes. Some examples are the flow in chemical reactors and combustion chambers, cloud dynamics, deposition of sediments on river banks, dust storms, etc. The behaviour of heavy particles or droplets in a turbulent flow has been thoroughly investigated over the past few years, see Snyder & Lumley (1971), Maxey & Riley (1983), L ´ azaro & Lasheras (1992b ), Schreck & Kleis (1993), Kulick, Fessler & Eaton (1994), Crowe, Troutt & Chung (1996), among others. There are, however, a number of issues that are still not well understood, the most important being the effect of the turbulence on the concentration field and on the settling of the particles, as well as the modification of the carrier flow turbulence due to the presence of the particles. It has been known for a long time that particles immersed in a turbulent flow tend to accumulate, creating large inhomogeneities in the concentration field. This preferential accumulation of particles in a turbulent flow has traditionally been explained by the inertial bias mechanism. When a particle that is heavier than the surrounding fluid interacts with a vortex, or a turbulent structure, the particle is accelerated in the outward direction by centrifugal forces. Auton, Hunt & Prud’Homme (1988), Ga˜ n´ an- Calvo & Lasheras (1991), Ruetsch & Meiburg (1993), Tio, Ga˜ n´ an-Calvo & Lasheras † On leave from: LEGI, CNRS/UJF/INPG, Grenoble, France.