Links between language, fluid dynamics, and the airflows that transport pathogens*

Growing evidence shows that the airflows that accompany speech during social interactions contribute to the transport of pathogens, such as the SARS-CoV-2 virus. However, it is still elusive how the manners of articulation during speaking, such as time-varying airflow rate, and instantaneous teeth and lip movements, may affect the transport features of airflow exiting the mouth. We combine experimental and numerical approaches to investigate the flow patterns produced by representative vowels, such as /a/, /o/, and /i/, and consonants, such as /p/, /k/, /s/, and /h/, which have distinct articulatory characteristics. A 3D vocal tract is modeled with a temporally varying exit that captures key morphologic and kinematic features of the human vocal tract, including teeth and lips, during speaking. An incompressible flow solver based on a sharp-interface immersed-boundary-method (IBM) is employed to compute the resultant airflow. By comparing representative utterance pairs, like /apa/ and /aha/, we are able to isolate the effect of articulatory features and show significant differences in spatial-temporal patterns of airflow under the influences of a time-varying orifice, different orifice aspect ratios, and different relative lip-teeth positions. This work helps bring insights into the understanding of articulatory phonetics, and the links to different languages, from a fluid dynamics perspective.