Free-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.
Acoustic Signatures of Pinch-Off Cavities During Water-Entry
College of Shipbuilding Engineering, Harbin Engineering University（哈尔滨工程大学船舶工程学院）；Nanhai Institute of Harbin Engineering University（哈尔滨工程大学南海研究院）；Southampton Ocean Engineering Joint Institute, Harbin Engineering University（哈尔滨工程大学南安普顿海洋工程联合学院）；National Key Lab of Ship Structural Safety, Harbin Engineering University（哈尔滨工程大学船舶结构安全国家重点实验室）
Water entry of small hydrophobic spheres on an axisymmetric wavefield
Brown University（布朗大学）；Georgia Institute of Technology（佐治亚理工学院）；École Polytechnique Fédérale de Lausanne（洛桑联邦理工学院）；Naval Undersea Warfare Center（海军水下战中心）
A two-dimensional analytical model of vertical water entry for asymmetric bodies with flow separation
考虑了非对称二维物体垂直入水时的分离流动。只要没有流动分离，线性化的Wagner理论与修正的Logvinovich模型相结合，已被证明可以在入水过程中提供计算快速和可靠的砰击载荷估计。Tassin等人（2014年）引入了Fictionary Body Continuation（FBC）概念，作为将Wagner模型的使用扩展到分离流配置的一种方式，但他们只考虑对称体。在本研究中，我们调查的能力，FBC的概念，提供准确的估计砰击载荷的非对称机构。在这种情况下，流动分离可能不会同时发生在主体的两侧。在中间阶段，砰击载荷由局部流分离引起的局部压降和润湿面积的持续膨胀之间的竞争决定。作为该模型的第一个基准，我们考虑了一个倾斜的平板的水进入和比较FBC估计与非线性模型的结果。然后，我们考虑的情况下，箔和比较FBC的结果与计算流体动力学的预测。在这两种情况下，我们发现，FBC模型是能够提供可靠的估计砰击载荷。
