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Impact force reduction by consecutive water entry of spheres.pdf
KAUST DepartmentPhysical Science and Engineering (PSE) Division
Online Publication Date2021-03-16
Print Publication Date2021-05-25
Embargo End Date2021-09-16
Permanent link to this recordhttp://hdl.handle.net/10754/668331.1
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AbstractFree-falling objects impacting onto water pools experience a very high initial impact force, greatest at the moment when breaking through the free surface. Many have intuitively wondered whether throwing another object in front of an important object (like oneself) before impacting the water surface may reduce this high impact force. Here, we test this idea experimentally by allowing two spheres to consecutively enter the water and measuring the forces on the trailing sphere. We find that the impact acceleration reduction on the trailing sphere depends on the dynamics of the cavity created by the first sphere and the relative timing of the second sphere impact. These combined effects are captured by the non-dimensional 'Matryoshka' number, which classifies the observed phenomena into four major regimes. In three of these regimes, we find that the impact acceleration on the second sphere is reduced by up to 78 % relative to impact on a quiescent water surface. Surprisingly, in one of the regimes the force on the trailing sphere is dramatically increased by more than 400 % in the worst case observed. We explain how the various stages of cavity evolution result in the observed alterations in impact force in this multi-body water entry problem.
CitationRabbi, R., Speirs, N. B., Kiyama, A., Belden, J., & Truscott, T. T. (2021). Impact force reduction by consecutive water entry of spheres. Journal of Fluid Mechanics, 915. doi:10.1017/jfm.2020.1165
SponsorsR.R., N.S., J.B. and T.T.T. acknowledge funding from the Office of Naval Research, Navy Undersea Research Program (Grant no. N000141812334), monitored by Ms M. Medeiros. A.K. is a JSPS Overseas Research Fellow.
PublisherCambridge University Press (CUP)
JournalJournal of Fluid Mechanics