  We investigate motion of a gas-liquid interface in a test tube induced by a
large acceleration via impulsive force. We conduct simple experiments in which
the tube partially filled with a liquid falls under gravity and impacts a rigid
floor. A curved gas-liquid interface inside the tube reverses and eventually
forms an elongated jet (i.e. the so-called a focused jet). In our experiments,
there arises either vibration of the interface or increment in the velocity of
a liquid jet accompanied by the onset of cavitation in the liquid column. These
phenomena cannot be explained by considering pressure impulse in a classical
potential flow analysis, which does not account for finite speeds of sound as
well as phase change. Here we model such water-hammer events as a result of
one-dimensional pressure wave propagation and its interaction with boundaries
through acoustic impedance mismatching. The method of characteristics is
applied to describe pressure wave interactions and the subsequent cavitation.
The proposed model is found to allow us to capture the unsteady features of the
liquid jet.
