Abstract-
Heavy particles moving in a turbulent field can be suspended for long times
despite the presence of the gravitational force. Due to inertia, heavy
particles tend to be ejected from vortical structures when those structures have a
smooth shape (as a sphere), end eventually fall down. In absence of gravity this leads to the
formation of clusters of particles in the strain regions, as described by Prof. Lance Collins in his
recent CES seminar.
However, eddies created in the turbulent energy cascade are characterized by multiple scales; in this situation the interplay between particle inertia and the curvature of the isovorticity surfaces can lead to suspension even in presence of gravity. As a consequence, individual heavy particles settling in a turbulent flow can have very different mean vertical velocities, favoring mixing processes over vertical lengths larger than the turbulent integral scales.
A hierarchy of models, from two-dimensional steady flows to direct numerical simulations of isotropic and homogeneous turbulence, is used to investigate the mechanism and its effects on cloud droplet settling rate. The mechanism is suggested as a possible candidate for partially explaining the presence of broad droplet size distributions commonly observed in convective clouds.
Energy field in a direct numerical simulation of isotropic turbulence. The sketch on the side is a cartoon to show the different effects of inertia on particle trajectories (black lines) depending on the curvature of the streamlines (blue lines).
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