Preferential concentration and settling of heavy particles in homogeneous turbulence

Abstract

Voronoı̈ diagrams are used to analyze one-way coupling direct numerical simulation

data of heavy particles settling in homogeneous turbulence. Preferential concentra-

tion and clustering of the inertial particles are analyzed for an extended range of

particle Stokes and Rouse numbers. Influence of preferential concentration on the

settling velocity enhancement is addressed from statistics of particle and flow field

quantities conditioned on the local concentration. While gravity is found to have

almost no influence on the global characteristics of preferential concentration, the

conditional statistics bring out a refined preferential sampling of the flow field resulting from the gravitational effects. This preferential sampling shows that beside

the descending fluid velocity contribution, the settling velocity is further increased

by the descending fluid acceleration. This effect cannot be detected from global estimations of the particle concentration field. A 2D analysis of the Voronoı̈ cells is also

presented to investigate their shape and orientation. It is found that clusters can be

represented as 2D elongated manifolds. Their shape is shown to be similar in zero

and non-zero gravity fields while Voronoı̈ cells tend to be more elongated for Stokes

numbers around unity. Under the gravity effects, they tend to be preferentially oriented perpendicularly to the gravitational axis