Optical properties and precipitation efficiency of atmospheric clouds are largely determined by turbulent mixing with their environment. When cloud liquid water is reduced upon mixing, droplets may evaporate uniformly across the population or, in the other extreme, a subset of droplets may evaporate completely, leaving the remaining drops unaffected. Here, we use airborne holographic imaging to visualize the spatial structure and droplet size distribution at the smallest turbulent scales, thereby observing their response to entrainment and mixing with clear air. The measurements reveal that turbulent clouds are inhomogeneous, with sharp transitions between cloud and clear air properties persisting to dissipative scales (<1 centimeter). The local droplet size distribution fluctuates strongly in number density but with a nearly unchanging mean droplet diameter.

Holographic measurements of the clustering of electrically charged, inertial particles in homogenous and isotropic turbulent flow reveal novel particle dynamics. When particles are identically charged, Coulomb repulsion introduces a length scale below which inertial clustering is suppressed such that the radial distribution function (RDF) mimics that of a nonideal gas. The result is described with a Fokker-Planck framework modeling inertial clustering as a diffusion-drift process modified to include Coulomb interaction. The peak in the RDF is well predicted by the balance between the particle terminal velocity under Coulomb repulsion and a time-averaged "drift" velocity obtained from the nonuniform sampling of fluid strain and rotation due to finite particle inertia. The resulting functional form of the RDF matches the measurements closely, providing support for the drift-diffusion description of particle clustering.

No abstract

No abstract

No abstract

No abstract

The presence of electrostatic charge can significantly alter the collision rate of inertial particles in turbulence. The influence of charge on the particle radial relative velocity and on the radial distribution function is investigated through direct numerical simulations of homogeneous, isotropic turbulence containing Lagrangian point particles. Particles with opposite charge polarity have enhanced inward radial relative velocity and radial distribution function, both increasing with decreasing separation distance. For like-charged particles, the converse is generally true. A simplified model for the influence of charge on relative velocity and radial spatial distribution is found to capture the general behavior. The model is based on the assumption of the superposition of relative velocity arising for charged particles in still fluid, and relative velocity arising from dissipation-scale turbulent velocity fluctuations.

No abstract