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

Natural processes, ranging from blood transport to planetary formation, are strongly influenced by particle collisions induced by background turbulence. While inertial clustering and particle pair relative velocity are recognized as the main collision enhancement factors, their phys-ical coupling is poorly understood. In this experimental study, we measure clustering and relative velocity in a polydisperse droplet field with background air turbulence, to directly demonstrate the physical coupling between these collision enhancement factors. This coupling is shown to cause an inverse relation between clustering and relative velocity in the mean-flow-dominated turbulent flow we study, thus suppressing the intuitive effect of an increase in droplet collision rate with background air turbulence. Turbulence modulation due to clustering, and the resultant reduction of caustic droplet pairs with large relative velocities, are found to be the key physi-cal mechanisms, and should be a consideration in droplet collision rate estimates in warm rain initiation.