Wakes of Freely Falling Water Drops

The study of 62 wakes of 2.9‐mm water drops falling through a cloud of small droplets revealed approximately cylindrical disturbances 5 to 10 drop diameters wide and 1340 diameters long for drop speeds equivalent to 78% of their terminal speed of 8.0 msec$\text{−1}$ . The wake disturbances lasted for about 9/16 sec at one place in space, in which time the air was displaced by about 0.5 cm in the direction of the moving drop. An evaluation of the flow pattern, based on velocity measurements showed that the averaged kinetic energy in the wake is extremely small and the work done by the drop on a layer of air with a cross section of the wake must be converted very rapidly into heat. The pattern and the decay of the measurable or gross vorticity, as well as the velocity gradients across the plane of observation are also given. The wake properties seem to agree with those of turbulent wakes; however, the development of a ringlike velocity maximum around the axis of symmetry is also possible. For rain it is concluded that the intensity of the small scale turbulence created by the falling drops is so small that, in general, no interactions are to be expected to affect their heat and mass transfer, their drag, or the collection of droplets by drops falling in the wake of another drop, at least for the wake ranges for which the results are applicable.