A Packet of Short Annular Perturbations around a Crown in the Initial Stage of Merging of a Free Falling Drop with a Fluid at Rest in the Impact Regime

The paper presents selections from a videofilm illustrating the rapid evolution of the fine flow structure during the merging of a water drop free-falling into a laboratory pool with tap water. In the initial stage of merging in the impact regime a packet of running periodic annular perturbations around the growing crown was visualized for the first time. The motion of the packet consisting of five to seven rings was traced according to the motion of a group of specks reflected from inclined sections of the fluid surface. The radial distances between the crests of perturbations in the packet (analogs of wavelengths) increase monotonically with the distance from the source. The time dependences of the lengths of individual components and the entire packet of annular perturbations, the velocities of its leading and trailing edges, and the frequency of disturbances at chosen points were determined. The dependences of the component dimensions on the frequency, as well as the cyclic frequency on the wave number of perturbations, were calculated. The conventional annular capillary waves of greater length are formed with a certain delay; they catch up the package of primary non-stationary disturbances and absorb them.