Experimental study on vertical water entry of two tandem spheres

In this study, the vertical water entry of two tandem spheres is experimentally evaluated through high-speed photography. It is found that because of the impact of the second sphere on the upward jetting formed by the surface closure of the first sphere, more kinematic energy is transmitted to the flow, forming a significantly large cavity behind the second sphere. As the second sphere descends into water, the cavity is elongated with a widely open water splash. For different impact velocities of the second sphere, the water splash shapes are obviously different owing to the pressure drop across the splash curtain. After the cavity deep seal, an interesting "multisection cavity" appears behind the second sphere due to disturbance of the bubbly wake by the first sphere. When the second sphere penetrates the bubbly flow caused by the cavity collapse of the first sphere, it moves relatively faster than a single sphere with the same initial impact velocity by reason of the bubble drag reduction effect, which is also verified by the quantitative data of accelerations for the spheres by a digital image correlation method.