Fluid Dynamics of Speech and the Spatial-Temporal Distribution of Aerosols

The spread of COVID-19 highlights the lack of guidelines and mitigation strategies for reducing the impact of a contagious virus in the absence of a vaccine. In fact, casual interactions, such as speaking, involving asymptomatic individuals are believed to play a significant role in virus transmission. Nevertheless, there are few quantitative studies of the influence of the phonetic features of speech on the spatial extent of the exhaled flows, nor are the corresponding airflows between a pair of speakers known. Such studies have largely been absent from the fluid mechanics and transport phenomena literatures, and even absent more generally from quantitative studies of public health. One approach for tackling this theme of "localized transport" of pathogen at the scale of individuals is to use model experiments representative of the airflows in speech along with numerical simulations based on the corresponding equations of fluid motion to characterize the spatial and temporal features of the airflows, with attention given, by incorporating knowledge in linguistics, to the phonetic-features characteristic of spoken English. This study will fill this important gap in the literature and allow broader thinking about the fluid dynamics of pathogen transmission between people, as well as animals which are important to the food supply. It will also provide insights necessary to assess risk of infection in models of casual interactions. The outreach efforts included characterize the investigator's approaches to engaging with, teaching, and mentoring future research scientists, including continuing an annual "holiday" lecture that has been delivered since 2002.

The goal of this project is to provide new qualitative insights and quantitative characterization of the airflows typical of conversations, as it is now better appreciated that airborne transport can, in many cases, be a major, if not the dominant, mechanism of viral/pathogen transmission. In fact, the realization of dangers posed by airborne spread of COVID-19 surprised the CDC and WHO for many months despite 100 years of research in public health since the 1918 pandemic. To address this challenge the research to be performed will integrate experiments and numerical simulations in three systematic aims. First, the development of flows in speech will be quantified by measuring pressure and flow rate signals characteristic of sounds and words, including the signatures of vowels, consonants, and plosives. The physics of the flow will be investigated in experiments using particle-image velocimetry and a laser-illuminated sheet of fog droplets around a speaker and corresponding image processing, and direct numerical simulations will solve the Navier-Stokes equations with initial and boundary conditions consistent with the experiments. Then, experiments and numerical simulations will be used to study flows in face-to-face conversations, including the key role played by the vertical offset between the source flows, and the corresponding spread of exhaled material. Finally, experiments and numerical simulations will characterize the influence of a nearby horizontal boundary, such as a table, which influences horizontal propagation of exhaled material. The results will be made available to other researchers who can then also take advantage of this database. The research themes will advance the understanding of pathogen transport that may accompany fluid flows of speech that are characteristic of casual interactions.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.