Motion dynamics of dropped cylindrical objects in flows after water entry

Abstract The investigation on the phenomena of objects falling into water is of great significance to protect the underwater equipment in offshore industry. The dropped objects into water are found to be affected by many factors such as drop angle, drop height, its physical properties, etc. This study mainly focuses on considering the influence of water environment (currents and waves) on the dropped cylindrical objects after water entry by modifying the three-dimensional (3D) theory (Xiang et al., 2017) and integrating the effects of currents and waves into DROBS. Current is added as a uniform steady flow. The waves will bring a non-uniform unsteady flow field based on 3D airy wave theory. Thus, analytic expressions for the hydrodynamic forces and moments are modified by adding inertial terms from waves. The drag forces are revised by rewriting the relative cylinder to water particle velocities. By the comparison between the simulated trajectory of the cylinder under currents and corresponding recorded trajectory in the experiments, the simulations agree well with the experimental data. The effects of speed and heading of the currents associated with the cylinder's initial speed are studied in DROBS and found to influence the trajectory and motions of the dropped cylindrical objects. Also, the dynamics of a dropped cylinder moving in the field of a progressive regular wave is investigated for different wave amplitudes, wave frequencies, and for different ratios of cylinder to water mass density.