A three-dimensional model of an impacting droplet

grantor: University of TorontoA three-dimensional model has been developed, to predict flow behaviour of a droplet impacting a solid surface. The model combines a fixed mesh discretization of the flow equations with a piecewise linear volume tracking algorithm to track the droplet free surface. Surface tension is modelled as a body force acting on fluid near the free surface. Contact angles are applied as a boundary condition at the contact line. Results are presented first of two asymmetric impacts, of a water droplet onto an incline and onto a sharp edge. Corresponding photographs are presented, of these and subsequent results. A simple model of contact angle variation with contact line velocity is proposed and implemented; simulation results compare well with the photographs. The model is then used to simulate droplet fingering and splashing, a manifestation of an unstable advancing contact line. A methodology is presented for introducing a perturbation of the velocity field near the solid surface, of prescribed amplitude and frequency, at a time shortly after impact. Simulation results are then presented of the impact of molten tin, water, and heptane droplets. Model agreement is good for a wide range of behaviours. An expression for a splashing threshold predicts the behaviour of the molten tin; the results of water and especially heptane, however, suggest that the contact angle plays an important role, and that the expression appears applicable only to impacts characterized by a relatively low value of the Weber number. The thesis concludes with application of the model to droplet impacts characteristic of the thermal spray process. First, results suggest that the splashing observed experimentally is not the result of the fluid instability under consideration here, but rather appears related to the solidification of material following impact. And finally, results are presented of the impact of thermal spray particles at various impact angles, to assess the influence of spray angle on splat geometry.Ph.D