Drop levitation in a turbulent, coaxial sheath flow

The behavior of raindrops has long been studied in vertical wind tunnels by both meteorologists and fluid dynamicist alike. Primarily, the study of raindrops in wind tunnels has been performed in near laminar conditions, but the behavior of raindrops in more turbulent conditions is less understood. Rather than strictly laminar flow, we levitate drops in a wind tunnel via the mixing of two coaxial flows. A slower core flow residing within a faster sheath flow provides sufficient drag and lift in order to levitate a liquid drop. By varying the ratio of the core flow to the sheath flow, we are able to augment the flow field and observe changes in drop behavior via a high-speed camera. In addition to water drops, we levitate drops consisting of water and glycerin to better understand the behavior of viscous drops. Using image processing, we are able to identify the multi-modal oscillations invoked by the flow field using Fast-Fourier Transform analysis as well as track the temporal changes in axis ratio. Our novel tunnel permits the study of a vast range of drop sizes, drop compositions, and drop behaviors such as oscillation, break-up, and coalescence in turbulent flow.