Influence of movement kinematics and buoyancy on the fluid dynamics of transmission in a waiting line.

Waiting in a line is a critical social interaction that occurs frequently in many public spaces. It represents a situation that could potentially increase the risk of transmission of respiratory viruses. Here we use a combination of laboratory experiments and direct numerical simulations (DNS) to assess the flow patterns created by periodic movements mimicking the kinematics of a waiting line. We matched the relevant non-dimensional numbers, including the Reynolds number of the line movement, and the Grashof number representing the buoyancy-driven fluid motion. UV-induced fluorescence, combined with particle imaging velocimetry, reveals the presence of competing counter-currents due to line kinematics and thermal gradients. We systematically explore the competing roles of the two using the DNS, which can either increase or decrease the risk of transmission depending on the pattern of movements and the temperature difference with the ambient.