Physical coupling between inertial clustering and relative velocity in a polydisperse droplet field with background turbulence

Droplet collisions in a turbulent air flow are understood primarily via two different enhancement factors, namely clustering and relative velocity. While these factors are believed to be physically coupled, they have mostly been studied independently, partly owing to challenges in fully accounting for droplet-turbulence interactions in theoretical models and numerical simulations. In this novel experimental study, we measure clustering and relative velocity in a controlled air turbulence facility, to directly demonstrate the physical coupling between these enhancement factors in a polydisperse droplet field. In the non-clustering regime, caustic droplet pairs, comprised of disparate droplet sizes, contribute primarily to an increase in average relative velocity (and hence collisions) with an increase in turbulent intensity. In the clustering regime, while caustic droplet pairs still occur, clustering and relative velocity share an inverse relation, thus contributing to a decrease in droplet collision rates with an increase in turbulent intensity. Turbulent modulation, and the resultant energy redistribution, are found to be the key physical mechanisms, and should be a consideration in droplet collision rate estimates in applications like warm rain initiation.