Cavity disturbance and impact modeling during water entry of a slender object with an inner flow diversion structure

The interaction of a passive jet guided through an inner flow diversion orifice of a water entry object with the cavity wall is a very interesting phenomenon. It can effectively reduce water entry impact by modifying the flow pattern without relying on external active control. In this study, we experimentally investigate the cavity disturbance, jet behavior, and impact force of a cylinder with a Y-shaped diversion structure impacting a quiescent free surface. The orifice allows part of the incoming liquid to be transformed into inner jets that induce cavity wall disturbance with different modes at different Froude numbers. The jet impact on the cavity wall induces the formation of an unsteady, chaotic cavity. It is worth noting that the stability of cavity development is related to both the Froude number and the diversion angle. We theoretically analyze the impact dynamics of the diversion structure based on added mass modeling and momentum transfer. It indicates that the orifice reduces the growth rate of effective added mass by tuning the momentum exchange path of the frontal liquid. A simplified model based on the dynamic pressure scale and the momentum flux of the internal diversion jet is also established. This model explains the main order of magnitude and variation trend of the peak acceleration difference between the solid model and the diversion model. The results confirm that the force reduction effect is due to the momentum flux release scale of the internal jet rather than the change in dynamic pressure area.