Atomisation of a thin liquid sheet by an impulsive air flow: the cough machine revisited.

We study experimentally and numerically the perturbation growth and the subsequent atomization of a thin liquid sheet by an impulsive airflow. The flow mimics the aerosolization of the muco-salivary fluid in the respiratory tract and revisits the "cough machine". We test both Newtonian and non-Newtonian fluids. Velocity ranges from ten to thirty m/s. In the Newtonian case, waves form that amplify into sheets. One sees bags inflating the sheerts. Holes then perforate these bags and leave behind a spray of droplets and ligaments. The depth, width, and thickness of the bags are correlated. Deeper, wider, and thinner bags are observed just before hole formation as viscosity is increased. The droplet size distribution is close to a log-normal PDF, and the geometric mean is decreasing with increasing fluid viscosity. In the non-Newtonian case, sheets are much more resilient and hole formation is difficult to observe. Simulations of the Newtonian case are performed using a VOF method with adaptive octree grid refinement, and a continuous surface force method with height function computation of curvature, all implemented in Basilisk (http://basilisk.fr). The simulations show qualitatively identical phenomena of sheet formation and hole formation and expansion.