An experimental study of the cavity evolution during the oblique water entry of a slender axisymmetric body in progressive waves is presented. The projectile, with a flat headform, is driven by a computer-controlled linear motor to ensure a constant impact velocity during the oblique water entry process. A high-speed camera is used to record the cavity during the oblique water entry. The regular wave is generated by a piston-type wavemaker. The linear motor, the high-speed cameras, and the wavemaker are synchronized to measure the cavity and surface elevation simultaneously. The influence of the location of the entry point on the cavity evolution is investigated. The experimental results show that, for the case of wave crest entry, the size of the cavity is the biggest. For the wave trough entry, the cavity evolution is similar to that of the zero-crossing point entry of the cylinder.
Water entry is a simple process with complex phenomena and exhibits transient, unsteady and nonlinear characteristics. Although the process of water entry is usually short, physical characteristics have distinct differences at different stages. Due to different requirements in engineering, the major concerns of the water entry of a body include prediction of impact loads on the body at the initial stage of water entry, understanding of the cavity evolution after the initial impact, and modeling the trajectory of the body during the water entry process. (Truscott et al, 2014)
The hydrodynamic problems associated with the water entry of a body belong to one of the classic subjects in marine hydrodynamics. The experimental study of the cavities during the water entry process dates back to the end of the 19th century. Using short-duration flash photography, Worthington and Cole (1897) studied the water entry of spheres and droplets through observing the shape of the cavity. Based on experimental observations, Bell (1924) presented qualitative discussion on the flow phenomenon and the mechanism of the rise, development and collapse of the cavity. Gilbarg and Anderson (1948) found that the surface seal of the cavity is affected by both the surface tension and the pressure difference between the inside and outside of the cavity. May (1975) presented a comprehensive review on the subject and reported the influences of Froude number and Reynolds number on the characteristic parameters of the cavity. Abelson (1970) measured the pressure distribution in the cavity and found that the pressure inside the cavity is basically the same before the cavity is closed.