Reduced particle settling speed in turbulence

TL;DR

This study investigates how turbulence affects the settling speed of particles, revealing that turbulence significantly reduces settling velocity due to increased drag and unsteady effects, with implications for understanding particle dynamics in turbulent flows.

Contribution

The paper provides new insights into how turbulence reduces particle settling speeds, highlighting the role of drag and unsteady effects in semi-dilute suspensions.

Findings

Settling speed decreases by 6%-60% in turbulence compared to quiescent fluid.

Vertical drag increases due to particle cross-flow velocity, reducing settling.

Unsteady effects contribute 6%-10% to overall drag.

Abstract

We study the settling of finite-size rigid spheres in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We study semi-dilute suspensions at different Galileo numbers, Ga. The Galileo number is the ratio between buoyancy and viscous forces, and is here varied via the solid-to-fluid density ratio. The focus is on particles that are slightly heavier than the fluid. We find that in HIT, the mean settling speed is less than that in quiescent fluid; in particular, it reduces by 6%-60% with respect to the terminal velocity of an isolated sphere in quiescent fluid as the ratio between the latter and the turbulent velocity fluctuations is decreased. Analysing the fluid-particle relative motion, we find that the mean settling speed is progressively reduced while reducing the density ratio due to the increase of the vertical drag induced by the particle cross-flow velocity. Unsteady effects contribute to the mean overall drag by about 6%-10%. The probability density functions of particle velocities and accelerations reveal that these are closely related to the features of the turbulent flow. The particle mean-square displacement in the settling direction is found to be similar for all Ga if time is scaled by (2a)/u' (where 2a is the particle diameter and u' is the turbulence velocity root mean square).