Vertical Oscillations of Water Droplets in the Supporting Vapour–Air Flow

Oscillatory Signatures in the Raindrop Motion Relative to the Air Medium with Terminal Velocity

“…It remains unclear why this or that frequency is revealed out of all the possible ones. Based on the previous studies [51,52,65], we are inclined to believe that droplet oscillations are of a self-excited nature since they were previously identified in the observations with droplet clusters within an absolutely laminar ascending flow. Nevertheless, it is also possible that the oscillations are partially related to external conditions, such as residual turbulence intensity < 0.5%.…”

“…In addition, the absence of considerable peaks in the spectra for the diameters means that the droplet walking from and toward the camera lens is small compared to the operational depth of the camera field. The camera is more tolerable of the object displacements out of focus at a large distance rather than at short distances when the diameter oscillations do contribute to the spectra [65].…”

“…Our experiment was performed in the Mainz vertical wind tunnel (Figure 1), in which water drops can be freely suspended at their respective terminal velocities [65] Outside air is sucked by two vacuum pumps through the wind tunnel. The air speed varies from 10 cm/s to 14 m/s with an accuracy of 1 cm/s in the observation section employed for these measurements.…”

“…For longer video recordings containing several 0.255 s segments, averaging was applied over the segments' spectra. This procedure is described in [65].…”