Air mass necking characteristics under vertical launching conditions

During vertical launching, after the underwater vehicle exits the launcher tube, an air mass forms that bridges the vehicle's bottom wall and the tube exit. This air mass undergoes necking and eventually ruptures, forming an attached tail cavity. This study conducts a numerical investigation on air mass necking, using an improved delayed detached eddy simulation model, volume-of-fluid method, zero-gap overset grid technology, and adaptive mesh refinement technology. The numerical method's ability to predict the tail and ventilated shoulder cavities is also verified. The fluid's constitutive equations are applied to analyze the shear forces acting on air mass necking. Results indicate that prior to the air mass rupture, it undergoes expansion, over-expansion, and contraction, driven by internal pressure. The necking point appears at the midstream of the air mass due to the wall constraint, starting with over-expansion. Additionally, the stationary launcher tube exerts a stationary shear force on the gas–liquid interface opposite to the vehicle's moving direction, causing the air mass to stagnate. The moving vehicle applies a dynamic shear force along the direction of movement, propelling the air mass forward. These shear forces cause the air mass's axial size to gradually increase, directly contributing to its rupture. When the attachment of a ventilated shoulder cavity is considered, initial high pressure inside the air mass leads to a large-scale cavity shedding from the rear of the shoulder cavity. This shedding cavity accelerates the onset of necking and the occurrence of oblique necking.