In two-phase flow, the interaction between multi-scale vortex structures and interfaces (bubbles or free surfaces) triggers a range of complex physical phenomena. This study employs numerical simulations to investigate the interaction between a horizontal vortex and the interface separating two layers of immiscible fluids with different densities (e.g., water and air). The vortex is initialized as an internal motion within the heavier phase. We focus specifically on the impact of the phase density ratio and surface tension. Numerical simulations reveal that when the density ratio is near unity, interface rupture occurs only at high Weber numbers ($\mathit{We}$), where low surface tension enables the rupture of sharp interface points. Conversely, at high surface tension (low $\mathit{We}$), these sharp points stretch into thin liquid films, significantly increasing the surface area without causing breakage. As the density ratio increases, interface rupture at sharp points accelerates, even under high surface tension, leading to faster dissipation of the initial vortex. In high-$\mathit{We}$ scenarios, an increased density ratio promotes the faster formation and greater intensity of new vortex layers at the interface. However, increasing surface tension enhances the vorticity of these layers but simultaneously slows their generation rate. The findings highlight the critical interplay between surface tension and density differences in vortex–interface interactions, with surface tension stabilizing the interface and density differences driving more intense vortex shedding and deformation. These insights offer valuable guidance for understanding two-phase flow behavior and improving the design of systems involving multiphase fluids.

This study investigates the hydrodynamic characteristics of supercavitating flows near boundaries through numerical simulations, focusing on the influence of free surfaces and solid walls. A validated numerical framework combining the Realizable k-ε turbulence model with the VOF multiphase approach and Schnerr–Sauer cavitation model is employed to capture complex phase interfaces and turbulent cavitating flows. Systematic validation against existing experimental data and dedicated underwater launch tests confirms the reliability of the methodology. The results demonstrate significant boundary effects on supercavitation dynamics. Near a free surface, the cavity tail deflects away from the interface, whereas it bends toward the wall when in close proximity to a solid boundary. Simultaneous presence of both boundaries causes combined torsional deformation and deflection. The cavity interface exhibits progressive indentation toward the tail, with indentation angle following a power–law relationship to axial position. Boundary interactions create distinct effects: free surfaces suppress cavity elongation, whereas solid walls promote extension, with cavity length remaining bounded within predictable limits. A notable torque reversal phenomenon occurs at $x=D$ from the cavitator tip, creating positive torque upstream (x < D) and negative torque downstream (x > D).

The wake characteristics behind a transom stern vessel play a crucial role in determining its hydrodynamic performance, resistance, and environmental impact. This hydrodynamic phenomenon involves violent wave breaking, posing significant challenges for experimental analysis. In this study, we explore the complex wake dynamics behind a transom stern vessel using high-fidelity three-dimensional numerical simulations. A sharp volume of fluid method is employed to capture the gas–liquid interface, while the immersed boundary method is applied to simulate the ship hull boundaries. A distinct advantage of the present simulation is the capability to conduct quantitative analysis within the turbulent two-phase mixing region characterized by significant air entrainment, which is difficult for traditional experimental and theoretical approaches. The research focuses on the interaction between free surface dynamics, air entrainment and turbulent vortex structures, which collectively shape the wake region. The main flow features of wakes, including wave patterns across various Froude numbers, air entrainment and the evolution of bubbly wakes, are investigated. Furthermore, the correlation between turbulent vortex structures and violent interface breaking is examined.