Sedimentation of finite-size spheres in quiescent and turbulent environments

TL;DR

This study uses direct numerical simulations to investigate how finite-size spheres sediment in quiescent and turbulent environments, revealing reduced settling velocities and different velocity distributions influenced by turbulence.

Contribution

It provides new insights into the sedimentation behavior of finite-size particles in turbulence, including velocity statistics and drag effects, using detailed numerical simulations.

Findings

Mean settling velocity is lower in turbulent flow by about 12-14%.

Particle velocity distribution is Gaussian in turbulence but has large tails in quiescent fluid.

Vertical dispersion is similar in both environments, but lateral dispersion is higher in turbulence.

Abstract

Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behavior of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using an Immersed Boundary Method to account for the dispersed rigid spherical particles. The solid volume fractions considered are 0.5-1%, while the solid to fluid density ratio 1.02. The particle radius is chosen to be approximately 6 Komlogorov lengthscales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reduction with respect to a single particle in quiescent fluid is about 12\% and 14\% for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails are associated to the intermittent fast sedimentation of particle pairs in drafting-kissing-tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behavior observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulas and show that non stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment.