Pendant drop rotation in vertical, turbulent, shearing flows

Abstract In natural and engineering applications, mobile drops frequently encounter nonuniform flows, and such interactions are both uncharacterized and diffcult to experimentally observe. In this study, we record the dynamics of pendant drops, a natural and ubiquitous drop state, subject to the non-uniform disturbances of aerodynamic turbulence and shear. Our turbulent, shearing flow is formed by a pseudo-coaxial jet. By protruding a co-flowing jet through the center of a vertical, suction-type vertical wind tunnel, we create a novel flow field termed sheathed, coaxial jet-flow (SCJF). Tuning the jet (core) and tunnel (sheath) velocities allows us to create turbulent, shearing flow fields that arise from the interactions at the flow interface. We employ an Eulerian-based finite-volume CFD solver within Star-CCM to characterize our novel flow and understand the drop dynamics therein. Subject to turbulent, shearing flows, pendant drops may rotate. The onset of drop rotation is influenced by turbulence below a threshold Reynolds number defined by the needle diameter. We find that pendant drop dynamics in SCJF are influenced by the size of the capillary support, the velocity ratio between the core and sheath, the drop size, and the magnitudes of turbulence intensity and shear imposed on the drop. We find that pendant drops exhibit limited modal behavior compared to that observed for laminar conditions and that small drops rotate faster than large drops. We form a scaling argument based upon the energetics of a rotating drop to reveal that dimensionless drop rotation, given by a modified Strouhal number, scales with an inverse drop Reynolds number. By comparing our results to the findings of our previous work in laminar flows, we discover that turbulence does not notably influence dimensionless drop rotation.