UC Berkeley

The COVID-19 pandemic heightened the interest in particle-laden turbulent jets generated by breathing, talking, coughing, and sneezing, and how these can contribute to disease transmission. Predicting the transport distance of the expelled droplets remains a critical, and still open, question. In the first segment of my work, I study the effect of ejection geometry and environmental boundary conditions on the transport of water density-matched particles. The study of particle transport from a canonical jet geometry illustrates the influence of secondary flows from the environmental boundary conditions (e.g. thermal gradients and wall effects) on particle dispersal. Subsequent experiments on the particle transport from a repeatable cough generator reveal that vortex structures created by the mouth and tongue structure have a large role in dispersal, due to the ejection of particles from regions of high vorticity.

In the second segment of my work, I study bubbly flows, in which the dispersed and carrier fluid phases are reversed. This flipped density ratio causes bubbles to accumulate in regions of high vorticity. Bubble transport is studied in the context of flow over a cylinder, for which the induced vibration is an important design consideration in devices from flow meters to nuclear reactors. While past researchers have shown that introducing bubbles to the flow can decrease vibration amplitude while increasing shedding frequency, the nonlinear shift in shedding frequency with bubble size has not been explained. Through experimental and numerical study of size-dependent bubble transport, I provide insight into the mechanismcausing this change in shedding frequency.

The study of both projects in tandem allows for the exploration of the effect of preferential accumulation/ejection on dispersed multiphase transport. In addition to complementary flow behaviour, the study of interchanged dispersed/carrier phases necessitates the development of highly specialized measurement techniques. I hope that the methods developed in this work see increased use in literature when developing validation datasets, as I look to challenge some of the common assumptions made in existing multiphase transport studies. A common theme through both projects is that typically second-order effects (e.g. spanwise velocity differences) have a significant influence on dispersed multiphase transport. As dispersed multiphase transport draws increased research interest, I hope that this work illustrates that (until proven otherwise) the collective understanding of the dominant mechanisms that exist for the single phase flow do not necessarily apply to multiphase transport.