Levitational Dynamics of a Droplet in a Vertical Wind Tunnel

In this study of liquid droplets, we study the simulation of stabilizing droplets for levitation within a vertical coaxial jet using Volume of Fluid (VOF)-based computational fluid dynamics (CFD). In our vertical wind tunnel setup, an inner upward jet is surrounded by an outer "sheath" flow, creating a zone where the droplet remains levitated by opposing gravity with a drag force, with minimal vertical oscillations. Using the CFD method, we simulate different droplet sizes and flow conditions to understand how forces like shear and pressure interact to hold the droplet in place. To maintain steady levitation, we implement a PID (Proportional-Integral-Derivative) controller that actively adjusts the inlet velocities dynamically based on the drop's current and previous position. Our results reveal insight into the underlying mechanisms of how a droplet's shape and movement change over time along with details coupling of the droplet and gaseous flows. Additionally, we probe the flow to quantify internal force distributions, coupling to turbulence, and mixing behavior. Results from these efforts provide new insight into droplets that are levitated that relates to cloud formation, combustion and more.