Research Projects

Flows of charged suspensions occur in natural and industrial settings. Such flows involve the interplay of particle inertia, turbulence, surface phenomena and electrostatic forces between the charge-carrying particles, and involve a wide range of length and time scales. We wish to understand the dynamics of charge separation and the resulting generation of electric fields. The primary mode of charge separation in such systems is through contact electrification when particles collide. Suspended particles of finite size have inertia, and do not necessarily follow fluid streamlines. In turbulent flows, suspended particles are expelled from regions of high vorticity, leading to ‘caustics’, which are regions of the flow where particles with very different velocities can come together. Large relative velocities lead to enhanced collision rates between particles. The caustics-induced increase of collision rates is a plausible resolution of the ‘droplet-growth bottleneck’ in warm-rain initiation, a puzzle that has remained unexplained for decades. The tribo-electric separation of charges is highly sensitive to surface properties; the magnitude of charge, but also the polarity of charging can depend on the surface properties of the particles colliding. The dependence of charge transfer on surface properties is a subject of active ongoing research. When the amounts of charge separated are sufficiently large, the resulting electric fields and electrostatic forces can themselves affect particle dynamics and, it has been suggested, may increase collision rates, thus forming a feedback loop in the dynamics. We propose to study, using direct numerical simulation (DNS) of the Navier-Stokes equations, turbulent flows of mono- and bi-disperse suspensions. We will track the Lagrangian trajectories of individual particles considering both one-way and two-way coupling with the turbulent flow. Using specific models for collisional charge transfer, we will study the evolution of the spatio-temporal evolution of the charge distribution, and numerically reconstruct the resulting electric field, accounting for its influence upon the motion of the charge-carrying particles directly and, through them, the turbulent flow. We will examine how the spatial structure of the electric field depends on the size distribution and the surface properties of the particles in the suspension, as well as the properties of the turbulent flow. We will apply our results to the study of charging in thunderstorms and volcanic ash plumes, with particle Stokes numbers St = O(0.01 - 10) and turbulent Reynolds numbers Re_{\lambda} = O(100 - 1000) . Other things being equal, our formulation can be used to infer the effects of surface properties on large-scale charge separation in turbulent flows in a wide range of natural and industrial applications.