Flow field structure and fluid dynamic characteristics of semi-closed cylinder water-entry

Numerical simulation for the vertical water-entry process of an end-closed cylinder shell was performed based on reynolds-averaged Navier-Stokes equations. The results of pressure and velocity contours, features of cavity wave and closure, rule of mass flow rate into the cavum, and hydrodynamic change rule were achieved. The influence of air compression-expansion on flow field structure and fluid dynamics was analyzed. The results show that water flow passing the open end in and out of cavum is synchronized with air movement; fluctuant pressure source appears at the open end of shell, leading to periodic pressure and velocity distribution; the expansion diameters of different axisymmetrical cavity sections are inversely proportional to speed of water into cavum; an air vortex generates in cavity, periodically enters and discharges from cavity, and prevents cavity closing; fluid dynamics present fluctuation characteristics whose frequency is equal to that of air, and the amplitude is in proportional relationship with that of air compression-expansion. The air compression-expansion motion in the progress of water-entry by the open cavum structure produces periodic perturbation of flow field and fluid dynamics, reduces impact of water-entry, and enhances the stability of cavity and motion of the an end-closed cylinder shell.