The effect of slenderness ratio on water entry

Abstract

In the paper, the air cavity created by vertical water entry of cylinders with different slenderness ratio (defined as “s=L/D”) is investigated experimentally, theoretically, and computationally. The study is focused on the range of Froude numbers, Fr = V/(gr) 0.5, (12< Fr< 30). Particular attention is given to the effect of slenderness ratios on the surface seal time, the evolution process of the cavities. To understand the water entry physical processes, we conduct several experiments of water entry of different length cylinders. A theory model considering the added mass (induced by the water motion), buoyancy force, gravity, and water-resisting force is developed to predict the relationship between the falling distance and evolution time. Studies show that for the same size bullet the surface seal time of the cavity decreases with the increase of impact speed. Under the identical impact speed, the cavity seal time decreases with the increase of the slenderness ratio. For the consistent impact speed, with the value of “s” increase, the falling distance increase (the same moment). The falling distance captured by the developed theoretical model match with the experimental and numerical data.