High-fidelity simulations of coughs and sneezes that serve as virtual experiments are presented, and they offer an unprecedented opportunity to peer into the chaotic evolution of the resulting airborne droplet clouds. While larger droplets quickly fall-out of the cloud, smaller droplets evaporate rapidly. The non-volatiles remain airborne as droplet nuclei for a long time to be transported over long distances. The substantial variation observed between the different realizations has important social distancing implications, since probabilistic outlier-events do occur and may need to be taken into account when assessing the risk of contagion. Contrary to common expectations, we observe dry ambient conditions to increase by more than four times the number of airborne potentially virus-laden nuclei, as a result of reduced droplet fall-out through rapid evaporation. The simulation results are used to validate and calibrate a comprehensive multiphase theory, which is then used to predict the spread of airborne nuclei under a wide variety of ambient conditions.

We study the body of turbidity currents in normal flow conditions by means of highly resolved direct numerical simulations of a homogeneous model. We focus on turbidity currents where the net amount of sediment is held fixed. We consider the sediment to be fine enough that their settling effect is neglected, and in the companion work we consider the effect of settling velocity. We consider five different shear Richardson numbers from 5 to 80. Under normal condition, basal drag and entrainment at the interface precisely balance the momentum supplied to the current from the excess weight of the sediment. The normal flow properties of a turbidity current can be fully characterized in terms of bulk Richardson number Ri and bulk Reynolds number Re. The velocity, concentration, and turbulent kinetic energy profiles take a self-similar shape when the current is at near-normal conditions. We observe the flow to display supercritical features for Ri ⪅ 0.4 and to display subcritical features for Ri ⪆ 0.7. From the behavior at intermediate Richardson numbers it appears that the transition between subcritical to supercritical behavior is not sharp. We observe good agreement between experimental and simulation results in both regimes. The entrainment coefficient as a function of bulk Richardson number at normal condition is in good agreement with the empirical relation and with available experimental results. We present a simple model for drag coefficient as a function of bulk Reynolds and Richardson numbers.