The mixing of airborne contaminants by the repeated passage of people along a corridor

We report a series of experiments in which a cylinder, with a vertical axis, is moved back and forth along a long narrow channel containing fresh water at Reynolds numbers Re = 3220–13 102. We examine the mixing of a cloud of dye along the channel by the oscillatory motion of the cylinder. Using light attenuation techniques to measure the time evolution of the concentration of dye along the channel, we find that at early times the concentration profile collapses to a Gaussian profile with dispersivity, D = (2.4 ± 0.5)fdW, where f is the frequency of the cylinder oscillation, d is the diameter of the cylinder and W is the width of the channel, respectively. For times much longer than L2/D, with L being the length of the channel, the concentration becomes progressively more uniform over the whole length of the channel, and we show that the long-time non-uniform component decays with time dependence exp(−4π2Dt/L2). We consider the implications of these experiments for the dispersal of viral aerosols along poorly ventilated corridors, with implications for infection transmission in hospitals and public buildings.