AN EXPERIMENTAL INVESTIGATION ON THE LOW SPEED VERTICAL WATER-ENTRY OF HOLLOW CYLINDERS

To reveal cavity evolution rules of the low speed vertical water-entry of hollow cylinders, an experiment was conducted incorporating high-speed video cameras and the visualization technique. According to the images of the flow patterns and motion parameters, the effects of hollow structures on cavity pinch-off, bubble evolution and velocity changes were addressed during the water-entry process. The results show that the deep-closed cavity at low water-entry speed collapses to an arc line between through-hole jets and cavitating flow instead of a point under the free surface. Therefore, the bubble shedding in steps, annular cavity on the jet and cloud cavity are observed at the stabilization stage of the cavity. The increase of water-entry velocity makes the cavity change from deep-closure to surface-closure with the cavity volume changed and different shedding patterns of the tail bubbles. The surface tension around the droplet presents minor effects on the overall motion of jets with a small bubble, which is approximately regarded as toss motion under gravity. The inflow in the hole contributes to the stability of the movement and the resistance curve has two types of changes and more inflection points with higher water-entry velocities.