  We experimentally study the airflow in a collapsing cavity created by the
impact of a circular disk on a water surface. We measure the air velocity in
the collapsing neck in two ways: Directly, by means of employing particle image
velocimetry of smoke injected into the cavity and indirectly, by determining
the time rate of change of the volume of the cavity at pinch-off and deducing
the air flow in the neck under the assumption that the air is incompressible.
We compare our experiments to boundary integral simulations and show that close
to the moment of pinch-off, compressibility of the air starts to play a crucial
role in the behavior of the cavity. Finally, we measure how the air flow rate
at pinch-off depends on the Froude number and explain the observed dependence
using a theoretical model of the cavity collapse.
