Fluid dynamics of transmission in a waiting line

Waiting lines represent a critical social interaction that occurs frequently in many public spaces, such as supermarkets, voting centers, COVID-testing centers and vaccination clinics. Here will simplify these complex scenarios into a fluid mechanical problem, with an objective of assessing person-to-person transmission in the waiting line. We use a combined experimental and numerical investigation to assess the flow patterns created by periodic movements mimicking the kinematics of a waiting line. Since field experiments can be challenging during the pandemic, we apply the concept of dynamic similarity to transform the waiting-line fluid flows to a laboratory setting (with water as working fluid). A conveyor belt system mounted with 3-D printed circular cylinders (dummies), driven by a stepper motor,  was used to model the people in the waiting line. We matched the relevant non-dimensional numbers: Reynolds number, reduced frequency, and dimensionless spacing to those of realistic waiting lines. UV-induced fluorescence, combined with particle imaging techniques, was used to study the unsteady flow patterns and the spreading of passive scalar. Some strategies will be  proposed to reduce cross-contamination in the waiting line setting — that go beyond the six-foot guideline currently in place.