Particles and snowflakes falling through turbulence
Abstract:The situation in which small heavy particles fall in a turbulent fluid (ubiquitous in nature and industry) is deceptively simple. Even stripped down to its minimal components, the problem remains complex because of the wide range of scales involved and the several physical parameters at play. In this talk, I will first focus on the case of dilute microscopic particles falling through homogeneous air turbulence. A unique experimental facility is leveraged, in which hundreds of jets are individually controlled to produce the largest volume of zero-mean-flow homogeneous turbulence ever created. Using high-resolution laser imaging, I will show how inertial particles group in larger clusters than previously thought, experiencing anomalously large accelerations and a multi-fold increase in fall speed. At volumetric concentrations of just 10 ppm, the particles also cause a substantial increase in turbulence intensity, at odds with most numerical simulations. I will then consider the case of heavy particles in turbulent boundary layers, studied in wind tunnel tests at the highest Reynolds numbers achieved to date. I will show how particle inertia fundamentally alters the classic paradigm by Rouse and Prandtl (postulated a century ago but still dominant in current models). The relevance of such observations is demonstrated by outdoor field measurements, in which snowflakes are illuminated and tracked by high-speed imaging over 30-square-meter vertical planes. The snowflakes display strikingly similar behaviour as inertial particles in the laboratory, including self-similar clustering, anomalous accelerations, and enhanced settling. These findings demonstrate that the fundamental phenomenology of particle-laden turbulence can be leveraged towards a better predictive understanding of snow precipitation. They also demonstrate how environmental flows can be used to investigate dispersed multiphase flows at Reynolds numbers not accessible in laboratory experiments or numerical simulations.
