Proceedings 2020, 49, 73; doi:10.3390/proceedings2020049073 www.mdpi.com/journal/proceedings Proceedings The Hydrodynamics of High Diving † Thibault Guillet 1 , Mélanie Mouchet 1 , Jérémy Belayachi 1 , Sarah Fay 2 , David Colturi 3 , Per Lundstam 3 , Peko Hosoi 2 , Christophe Clanet 1 and Caroline Cohen 1, * 1 LadHyX, École polytechnique, 91128 Palaiseau, France; thibault.guillet@polytechnique.edu (T.G.); mouchet@polytechnique.edu (M.M.); belayachi@polytechnique.edu (J.B.); clanet@polytechnique.edu (C.C.) 2 Department of Mechanical Engineering, MIT, Cambridge, MA 02139, USA; scfay@mit.edu (S.F.); peko@mit.edu (P.H.) 3 RedBull, Santa Monica, CA 90404, USA; dcolturi@gmail.com (D.C.); per.lundstam@redbull.com (P.L.) * Correspondence: cohen@ladhyx.polytechnique.fr; Tel.: +33-169335260 † Presented at the 13th Conference of the International Sports Engineering Association, Online, 22–26 June 2020. Published: 15 June 2020 Abstract: Diving consists of jumping into water from a platform, usually while performing acrobatics. During high diving competitions, the initial height reaches 27 m. From this height, the crossing of the water surface occurs at 85 km/h, and as such it is very technical to avoid injuries. Major risks occur due to the violent impact at the water entry and the formation and collapse of the air cavity around the diver. In this study, we investigate experimentally the dynamics of the jumper underwater and the hydrodynamic causes of injuries in high dives by monitoring dives from different heights with high-speed cameras and accelerometers in order to understand the physics underlying diving. Keywords: sports hydrodynamics; cliff diving; injuries; water entry; air cavity 1. Introduction Diving consists of falling or jumping through air into water from a platform, usually while performing acrobatics. During high diving competitions, the initial height reaches 27 m. From this height, the crossing of the water surface occurs at 85 km/h and as such it is very technical to avoid injuries [1–5]. Major risks occur from the violent impact at the water entry and the formation and collapse of the air cavity around the diver, if his body is not perfectly vertical and stiffened [6,7]. For Olympic competitions, the initial heights are 1, 3 or 10 m. In these cases, the athletes can enter the water with their arms forwards [8]. In a high dive, athletes always enter feet first and are limited to less than ten dives a day to avoid injury. It is well known among them that a transition occurs at 14 m, the height at which diving arms first becomes dangerous. Finally, the maximal height from which an athlete can jump into water without injury (feet first) is 58 m. In this study, we investigate experimentally the dynamics of the jumper underwater and the hydrodynamic causes of injuries in a high dive, by monitoring one male top-level cliff diver, jumping from different heights during his habitual training with high-speed cameras both in air and underwater and with an accelerometer fixed on his body. The final goal of this work is to better understand the physics underlying high cliff diving as inspired by the studies of the water entry of projectiles [9–11] and to be able to protect divers by monitoring their dives with embedded sensors or by designing safety gear.