Water entry cavity regime and impact mitigation of axisymmetric body with annularly distributed flow diversion orifices

The mitigation of water entry loading is of great significance in aerospace and marine applications. This study performs water entry experiments utilizing transparent resin axisymmetric models with in particular designed radial flow diversion orifices distributed annularly, to investigate the specific water entry cavity regime, flow field structures, wake features, and impact loads. Under different Froude numbers, comparative analyses were conducted between the models with and without diversion orifices. The results indicate that the model with diversion orifices facilitates the diversion of water from the impact zone into the internal channel of the orifice, developing into a distinctive radial jet interacting with attached cavity. The diversion orifices not only alter the impact region but also modulate the model's kinematic flow field through the action of the fluid within the orifices, thereby markedly reducing the water entry induced loads. Collisions between the fluid at orifice sites and the ambient water generate clusters of bubble clouds, which subsequently axially transfers along the model surface toward the free water surface. Periodic bubbly wake is generated and extended all the way to the water surface, distinct from the conventional pinch-off and deep seal regime. The annularly distributed radial jets induce a flow bifurcation that establishes dual-domain velocity vector fields from the particle image velocimetry result. Moreover, proper orthogonal decomposition captures and analyzes the wake structure. All the first-order modes of the models exhibit cavity expansion, which manifests as vertical oscillations in the water entry images. Starting from the second-order mode, only the 12 mm model manifests remarkable cavity detachment and wake structures resembling columnar vortex shedding.