The Settling Rates of Particles in Rayleigh–Bénard Turbulence

It is widely recognized that better understanding the interaction between clouds and aerosols is among the most pressing and essential endeavors in climate and atmospheric sciences. A particular process of interest is the settling rate of heavy particles and how it affects rain formation and droplet size distribution. While much work has been done to understand particle settling rates in homogeneous isotropic turbulence and the potential for particles to settle significantly faster than their predicted Stokes settling velocity, more work needs to be done in other environments. Towards this aim, a set of experiments was conducted utilizing the Pi Cloud Chamber experimental facility at Michigan Technological University to investigate the physics dictating particle settling time in Rayleigh-Bénard turbulence.

Among other features, the Pi Chamber can maintain a statistically steady, turbulent Rayleigh-Bénard flow at a Rayleigh number of approximately 10⁹. In this set of experiments, solid, non-evaporating spheres (namely oil, glass, and hollow glass) of varying sizes were added to the chamber. After the initial injection, the decreasing concentration of particles over time was measured, and a decay rate was calculated from this data. An ideal Stokes settling velocity would predict that these decay rates vary as the square of the particle diameter, and a primary goal of this work is to quantify any deviation from this theoretical behavior. In this talk, I will share the results according to particle material and size, highlighting the multiple physical mechanisms dictating settling rates. Further, I will compare these results with direct numerical simulations of the performed experiments. In future work, this knowledge will be combined with condensational growth and decay in a cloudy environment to improve our understanding of cloud droplet growth and size distribution evolution.