Tuning body shape and stiffness to mitigate water-entry forces

Abstract

High-speed water entry of projectiles and diving systems induces high forces and jerk to the entering bodies due to the development of large hydrodynamic pressure. Previous research has shown separately that the peak forces can be reduced by improving the aerodynamic shape of the head (nose) or, recently, by introducing a spring element between the head and body. This study seeks to understand whether the aerodynamic shape or spring stiffness coupling is most important for force reduction by combining both in one study. The experiment combines the nose cone aerodynamics and spring stiffness with a rear body and examines the forces acting on the nose and body. Three parameters are varied: the nose angle, spring stiffness, and impact velocity. An unsteady semi-analytical formulation is developed to estimate the water entry forces and coupled body dynamics. We find that the peak force reduction due to the spring is highest when the slamming force is most significant, particularly at higher impact velocities and with blunter nose angles. The spring coupling enables periodic fluctuations between the kinetic and potential energy throughout the duration of impact, which can be tuned by varying the stiffness. These findings can allow engineers to control the dynamic response of water entry.