Water entry dynamics of hydrophobic spheres bounded by semi-infinite cylindrical pipes

We advance the important topic of water entry by documenting splash dynamics and changes in hydrodynamic drag arising from the transport of free-falling hydrophobic spheres through semi-infinite cylindrical pipes. Water entry through narrow, confined spaces is yet another means to manipulate splash behavior and sphere transport. In this experimental study, we investigate the fluid–structure interactions between the walls of semi-infinite cylindrical pipes and descending hydrophobic spheres for Froude numbers in the range of 21–97. Generally, impacts arising from the water entry of unbounded hydrophobic spheres produce radially expanding splash crowns above the surface and smooth axisymmetric air-entraining cavities below the water line. We find cavity-producing impactors traversing narrow cylindrical pipes experience higher splash crowns, shallower cavities, and increased hydrodynamic drag when compared to entry into an unbounded liquid bath. Thus, we show that the tuning of entry dynamics is dependent on pipe diameter. Our study unravels the physics behind the often-encountered phenomenon of solid projectiles navigating confined spaces, and our results translate to passive, non-propulsive underwater transport related to industrial and commercial activities.