AIR ENTRAPMENT BY A FALLING WATER MASS

The impact of a nearly cylindrical water mass on a water surface is st udied both experimentally and theoretically. The experiments consist o f the rapid release of water from the bottom of a cylindrical containe r suspended above a large water tank and of the recording of the free- surface shape of the resulting crater with a high-speed camera. A bubb le with a diameter of about twice that of the initial cylinder remains entrapped at the bottom of the crater when the aspect ratio and the e nergy of the falling water mass are sufficiently large. Many of the sa lient features of the phenomenon are explained on the basis of simple physical arguments. Boundary-integral potential-flow simulations of th e process are also described. These numerical results are in fair to g ood agreement with the observations.