Wall effects induced by semi-infinite cylindrical pipes in hydrophobic sphere entry

We advance the important topic of water entry by documenting cavity dynamics and changes in hydrodynamic drag arising from the transport of hydrophobic spheres through semi-infinite cylindrical pipes. Water entry through confined spaces is yet another means to manipulate cavity behavior and sphere transport. In this experimental study, we investigate the fluid-structure interactions between the walls of semi-infinite cylindrical pipes and hydrophobic spheres for Froude numbers in the range of 20 - 100. Generally, impacts arising from the water entry of hydrophobic spheres produce smooth axisymmetric cavities below the water line. We find cavity-producing impactors traversing water-filled cylindrical pipes experience changes to cavity pinch-off, cavity shape, and hydrodynamics drag when compared to entry into an unbounded aqueous pool. The tuning of cavity behavior is dependent on Froude number and pipe diameter. Our study unravels the physics behind the often-encountered phenomenon of solid projectiles navigating confined spaces and our results translate to the entry dynamics of passive, non-propulsive underwater transport related to industrial and commercial activities.