Free surface flow focusing

Reducing the area through which a fluid is allowed to flow often leads to an increase of flow velocity. A familiar example of this is a garden hose, where one can change the rather weak stream that usually flows out into a strong jet by narrowing the orifice at the end of the hose. In this example, the walls of the garden hose orifice are forcing the water to flow through a smaller area, and hence increase the velocity. A similar phenomenon happens naturally after an axisymmetric object (like a circular disc or a sphere) impacts on a water surface. A surface cavity is formed, which collapses and after pinch-off two jets are formed: one shooting upwards and one downwards. The velocities of the jets and also of the water surface just before the pinch-off are usually much higher than the velocity of the impacting object. This is because the free surface evolves in such a way that it focuses the liquid towards the axis of symmetry of the system. We show that not only the liquid phase, but also the entrained air attains high velocities. A subtle interplay between the air flowing out and the water rushing in, eventually leads to an air flow that attains supersonic speeds. By impacting discs with a non-axisymmetric disturbance, we experimentally show that asymmetries have a dramatic effect on the evolution of the collapsing cavity. Perturbations that initially exist - no matter how small – persist throughout the collapse in an oscillating manner, with growing relative amplitude. As a consequence, non-linear effects start to influence the shape of the cavity. These findings, and others that are presented in the thesis, can be used in, for example, research into inertial fusion energy and the formation of stellar jets.