NUMERICAL INVESTIGATION ON THE WATER-ENTRY CAVITY FEATURE AND FLOW STRUCTURE OF A SPINNING SPHERE BASED ON LARGE-EDDY SIMULATION

The water-entry process of spinning sphere has great significance to the research of the up-to-date load reduction method of water-entry based on pre-launched object. In the present work, large-eddy simulation method is used together with the homogeneous multiphase flow model and VOF algorithm of interface capturing, to simulate the water-entry free motion of a fast-spinning sphere with hydrophobic coating at low Froude number, thus to investigate the water-entry cavity evolution, the flow structure and the hydrodynamic features. The free motion of the sphere is achieved through the dynamic mesh and sliding mesh techniques. The reliability and accuracy of the numerical simulation results are validated by comparison with previously published experimental results with good agreement on the transient cavity shape and the motion of the sphere. The spinning motion induces a lift force on the sphere and the trajectory of the sphere has significant curvature along its descent. A persistent wedge of fluid is emerged across the center of the cavity due to the fluid along the surface dragged by the sphere. The velocity and spin rate were normalized with the impact velocity and spin rate to analyze the numerical results. It shows that the spin rate has significant influence on the cavity evolution and hydrodynamic characteristics. Both of those cavity shapes have asymmetrical splash curtain and collapse asymmetrically. As spin rate increases, the horizontal velocity and the maximum lift force increase, while the maximum lift force is also limited by the impact velocity. The spin rate increase also leads to a stronger wedge of fluid forming. As a result, the pinch-off pressure maximum decreases and less vortex structures are observed. And also, the spin rate increase leads to lower side pressure during the initial impact phase. However, the vertical dynamic characteristics of spheres, like vertical velocity, acceleration and immersion depth of pinch-off, are less affected by the spin rate. Moreover, the sphere spin rate is less affected by the impact spin rate increase before cavity pinch-off.