Tuning body shape and stiffness to reduce water slamming forces

Abstract

This paper reveals how plunge-diving seabirds control impact energy during high-velocity water entry to hunt fish in deep waters without breaking their necks. Previous research has shown that the aerodynamic shape of the head or the structural compliance in the neck can reduce slamming forces. However, the physics governing their combined effects combined on the dive performance are is not well understood. The paper addresses this gap by demonstrating analytically and experimentally why the combined effect of shape and compliance is key for controlling the energy transmission during impact, passively. The impact forces at varying velocities are measured experimentally using a simple projectile design— to emulate seabirds’ dives —with different head shapes (cone angles) and spring stiffnesses (compliance). The experiments are utilized to develop a semi-analytical model to estimate the amount and duration of the stored, released, and dissipated energy. Our findings show that the slamming forces can be passively reduced by tuning the compliance to increase the amount of impact energy stored in the system and delay its release and dissipation. While decreasing the cone angle reduces the slamming forces for a rigid system, the effect of compliance on reducing these forces is more pronounced in projectiles with half-cone angles larger than 30°. Modeling the interplay between cone angle and neck compliance offers physical insights into how diving seabirds mitigate mechanical stresses during impacts, thereby avoiding catastrophic damage. Conversely, these insights can be exploited to engineer mechanical systems with passive control of dynamic loads such as impact, shock, or vibrations with minimal energy losses.