Single and double drop impacts onto deep and thick liquid layers

The present experimental and analytical study deals with single and double drop impacts onto a deep pool or liquid layers of finite thickness, with a general purpose of understanding the complex flows generated by spray impacts. The study is focused mainly on the observation of the evolution of the crater formed by the drop impacts and the description of the associated flow in the liquid layer. A test rig for the high-speed-visualization of interface phenomena has been built and experimental investigations using highspeed-imaging and image processing have been performed. Single drop impacts onto a deep pool have been studied in relation with the extensive existing literature. A new classification of the flow regimes has been suggested and a new “trampoline” regime has been detected and described. The phenomenon of the closure of the crown above the crater, typical of terminal speed impacts, has been obtained with unprecedented low impact parameters and it has been observed to be promoted by the sphericity of the impacting drop, while it does not take place if the drop is oblate. A phenomenological investigation of double drop impacts onto a deep pool has been for the first time performed. The phenomena arising have been observed to be characterized by several interactions, which have been described and discussed. The two craters and the two jets ejected from the craters may merge each other. New complex hollow shapes of the crater or of the jet resulting from the merging may be produced and a new mechanism of bubble entrapment has been detected. A test case for the validation of numerical codes has also been proposed, since the simultaneous impact of two identical drops represents a new example of three dimensional free surface flow, instead of the axial symmetry typical of a single drop impact. The influence of a finite value of the depth of the target liquid layer has been investigated in comparison with single drop impacts onto a deep pool, with particular attention on the crater evolution. The presence of the wall has been observed to reduce the velocity of penetration of the crater, but to do not significatively change its maximum depth, which is only reached later. For this reason the maximum crater depth of a deep pool impact has been chosen as reference parameter for the usual classification in thin films, thick films and deep pools of the depth of the target liquid layers in relation with the parameters of the impacting drop. This choice has a more physical significance, since it accounts for the kinetic energy of the impacting drop instead of only its diameter. The crater formed by the impact of a drop onto a deep pool has been characterized for several impact parameters. A new theoretical model for its evolution has been formulated. Potential flow theory has been used to model the flow around the crater as the velocity field given by a moving expanding sphere, whose equations of motion have been obtained through a balance of stresses at the crater interface and include the effects of inertia, gravity, surface tension and viscosity. Agreement with experimental data from the present study and from literature is rather good. A map of the predicted values of the maximum crater depth as a function of the impact parameters has been produced.