Journal of Fluid Mechanics
Cambridge University Press
The forces on an object impacting the water are extreme in the early moments of water entry and can cause structural damage to biological and man-made bodies alike. These early-time forces arise primarily from added mass, peaking when the submergence is much less than one body length. We experimentally investigate a means of reducing impact forces on a rigid sphere by placing the sphere inside a jet of water so that the jet strikes the quiescent water surface prior to entry of the sphere into the pool. The water jet accelerates the pool liquid and forms a cavity into which a sphere falls. Through on-board accelerometer measurements and high speed imaging, we quantify the force reduction compared to the case of a sphere entering a quiescent pool. Finally, we find the emergence of a critical jet volume required to maximize force reduction; the critical volume is rationalized using scaling arguments informed by near-surface particle image velocimetry (PIV) data.
Speirs, Nathan B. et al. "The Water Entry of a Sphere in a Jet." Journal of Fluid Mechanics. Cambridge University Press, 2019, vol. 863, pp. 956-968. Published online 29 January 2019. doi: https://doi.org/10.1017/jfm.2018.931