Non-Stokes deposition velocities observed under Rayleigh–Bénard turbulence

The settling and deposition behavior of small particles significantly impacts many environmental and industrial processes. Although particle settling dynamics within turbulent environments have been extensively studied through theory and simulation, more experimental investigation is needed to test the idealized results against real-world turbulent flows. In this work, we infer the settling velocity of particles in Rayleigh–Bénard (RB) turbulence through the measurement of declines in particle concentration due to deposition within a 3.14 m3 convection chamber. Oil droplets from 1 to 10 μm and glass beads from 1 to 38 μm were used as the particle material, yielding Kolmogorov-based particle Stokes numbers in the range of 3×10−5 to 0.1. Our measurements reveal a linear scaling with particle diameter, rather than the diameter-squared scaling expected for these Stokes numbers. Direct numerical simulations (DNS) designed to mirror the experimental configuration were also performed; these yielded the familiar diameter-squared scaling. We suggest that the discrepancy between the experimental results and the theoretical framework and DNS, which makes standard assumptions regarding lift and drag, may be due to unresolved physics near the domain boundary, which dictates overall removal rates. Future work employing more direct observational methods of particle settling within RB turbulence is required to better understand these near-wall removal processes. The departure from Stokes settling within this set of experiments brings to question the limits of the applicability of the “stirred settling” model commonly invoked for estimating particle removal rates, suggesting that adjustments to traditional assumptions may be required. Such corrections may prove especially important in the design of future cloud convection chambers.