Study on the high-speed shallow-angle water entry of cylinders with varying densities

Abstract

An investigation into the transient fluid-structure interactions during the high-speed (600 m/s) shallow-angle (8°) water entry of cylindrical projectiles with varying densities (2.7-16.1 g/cm³) is presented. The fundamental mechanisms governing cavity dynamics and projectile stability are revealed using three-dimensional computational fluid dynamics (CFD) simulations, which are validated by synchronized high-speed imaging. The key findings demonstrate that asymmetric wetting of the cylinder’s head during the early stage of water entry induces a critical head-down moment that governs subsequent hydrodynamic behavior. Three distinct fluid dynamic mechanisms are identified: 1) Delayed upper cavity formation accompanied by asymmetric cavity expansion; 2) Splash convergence producing distinct upward and downward jets, with the latter inducing localized cavity collapse upon impacting the wall; and 3) Pressure redistribution at the head end caused by variations in angle of attack, which generates restoring moments through asymmetric flow patterns. Density-dependent kinematic analysis reveals that within the same range of horizontal displacement, low-density cylinders (ρ≤4.1 g/cm³) undergo multiple tail slaps, whereas high-density cylinders (ρ≥7.2 g/cm³) achieve rotational stabilization through head-end restoring moments prior to tail slap initiation. Quantitative analysis shows that increasing the density from 2.7 to 16.1 g/cm³ reduces the maximum angular deflection by 89.43 % and the accumulated trajectory curvature by 42.83 %. These findings establish material density as the primary control parameter for ricochet prevention during shallow-angle water entry.