CyberDiver: an untethered robotic platform for water-entry experiments*

High-speed water entry of solid bodies has been a subject of intense investigation for over a century, featuring rich multi-scale physics and high relevance to engineering applications. Several recent works have extended the classical understanding of water entry dynamics – in which impactor speed and geometry are the primary independent variables – to the case of a flexible impactor with one or more elastic degrees of freedom, showing that flexibility can significantly influence the impact force. The CyberDiver enables the further possibility of not just passive, but active motion during water entry experiments, inspired by examples in the biological world such as sea birds that tuck their wings at impact or Olympic divers performing a "rip" entry. The CyberDiver is simplified to a single axial degree of motion that is driven by a linear motor and dropped into a quiescent water bath with normal incidence. The relative position and coupling force between an axisymmetric nose piece and cylindrical trailing body can be prescribed in relation to the moment of impact, and the resulting impact force, splash and cavity dynamics analyzed. Furthermore, high-bandwidth onboard sensing of the nose displacement and coupling force enables the CyberDiver to operate as an untethered cyber-physical system, permitting an arbitrary structural coupling that is designed in software to be experimentally tested and exposed to the complete set of impact physics.