Disks settling in turbulence

Abstract

This paper describes an experimental investigation of the dynamics of freely falling thin circular disks settling through turbulence. The patterns of the three dimensional disk motion are studied using an orthogonal arrangement of two high speed cameras. Turbulence is generated in a water tank using a random jet array facility where the jets are mounted in a co-planar configuration. The facility is run continuously until turbulence reaches a statistically stationary state, then, all water pumps are turned off simultaneously and a disk is released after a given waiting time. Contrary to spherical particles, finite-size inertial disks show an increase in the descent velocity for turbulence velocity fluctuations smaller than the particle descent velocity in quiescent flow. Thus, we observe a severe increase of the mean descent velocity of the disk with increasing the magnitude of the turbulence velocity fluctuations (up to $20\%$ of the velocity in quiescent flow for the disk with higher dimensionless inertia $I^*$). We also observe descent events that do not exist for disks falling in still fluid; i.e. `slow tumbling' events and `levitating' events. Finally, we show that the dominant frequency of the particle oscillatory motion decreases for increasing descent velocity and that particles exhibit oscillatory frequencies that never exceed the dominant frequency in quiescent flow in more than $30\%$.