Zijie Zhao

Passive source localization is essential in underwater acoustics, yet both model‑based matched field processing (MFP) and conventional machine learning methods often suffer from limited accuracy and poor generalization. To address these challenges, this study presents a low-frequency underwater acoustic ranging approach that integrates data augmentation with the ResNet-UNet architecture. Using the real and imaginary components of the covariance matrix as inputs, the sample expansion is first performed by combining the deep convolutional generative adversarial network with several conventional augmentation techniques. Afterwards, a predictive model that fuses ResNet and U-Net is developed for target range estimation. The validity of the proposed method is examined through the SWellEX-96 sea trial data, where the performance is compared under two conditions, with and without the augmentation strategy, and also benchmarked against several reference methods, namely MFP, generalized regression neural network (GRNN), conventional convolutional neural network (CNN), ResNet, and the proposed ResNet-UNet. Experimental results indicate that the adopted augmentation can considerably enlarge the training sample set, which consequently enhances the ranging accuracy. The majority of MFP estimates fall beyond the acceptable error margin, while GRNN shows obvious weaknesses in generalization performance. Both the conventional CNN and ResNet are only capable of producing coarse range approximations. Nevertheless, when coupled with the proposed augmentation, the ResNet-UNet method effectively accomplishes range estimation and its performance markedly surpasses that of the other models. Moreover, it remains effective even under low low signal-to-noise ratio conditions.

This study employs a combined experimental and numerical approach to investigate the spatiotemporal evolution of underwater supersonic gas jets and the influence of varying expansion ratios. Experiments were conducted in a large-scale open water pool, utilizing high-speed photography to capture the complete evolution process. Simultaneously, numerical simulations were performed under identical conditions using the volume-of-fluid method coupled with the shear-stress transport k–ω turbulence model. The numerical results exhibit excellent agreement with experimental data regarding the gas mass's morphological transitions. Results indicate that the underwater jet's initial development can be delineated into three distinct stages: initial gas expansion, interface mixing and primary necking, and continuous development. Analysis of the Mach number and temperature contours reveals the secondary gas mass emerges primarily due to the stagnation of the jet's axial motion caused by the head gas mass's tail interface, alongside the exhaust plume's compression by the liquid reentrant jet. Furthermore, larger expansion ratios lead to a higher degree of shock field confinement. Conversely, smaller expansion ratios yield higher axial momentum, suppressing Mach disk formation within the core jet region. Pressure monitoring identifies the primary oscillation region at 5De downstream of the nozzle exit. Simulations also capture typical Kelvin–Helmholtz instability structures, demonstrating that the synergy between the pressure field and interfacial instability drives periodic pressure oscillations following primary necking in under-expanded conditions. These findings provide significant guidance for the nozzle design of underwater solid rocket motors and offer a robust theoretical foundation for understanding compressible multiphase interactions.

In realistic ocean environments, the random propagation direction of waves leads to the misalignment with the water-entry trajectory of vehicles under oblique wave condition, and a critical yet underexplored factor of wave-to-course angles is introduced into the cavity dynamics. This study investigates the cavity evolution during the oblique water entry of a cylinder into waves via detached eddy simulations across four wave-to-course angles, where the numerical method is validated against the experiments conducted in a wave flume. For the head-wave entry with wave-to-course angle of 0°, the splash crown rapidly closes on the cylinder surface, producing an axisymmetric cavity. As the wave-to-course angle increases, the relative angle and distance between the cylinder and the local wave surface decrease. This promotes the formation of a low-pressure water film on the wave-facing side, and the resistance to cavity expansion toward the free surface is reduced. Consequently, cavity closure is delayed until the surface, which results in a larger and markedly asymmetric cavity oriented toward the wave. Accordingly, the primary mechanism for the effect of wave-to-course angles on cavity evolution is identified as the angle between the cylinder and the wave surface, indicated as the actual water-entry angle. Based on this, a generalized model for the actual water-entry angle is established, incorporating the wave-to-course angle, water-entry angle, and wave phase at impact. This model extends the wave-induced water-entry analyses to arbitrary combinations of these parameters, providing a foundational framework for predicting cavity dynamics in complex ocean environments.

During the high-speed water exit of vehicles at shallow angles (θ<20°), prolonged interaction with the free surface significantly enhances the influence of geometric and initial motion parameters on cavity evolution and trajectory stability. In this study, a numerical method coupling three-degree-of-freedom motion equations with the volume of fluid model investigates the mechanisms of nose shape and water-exit angle on trans-media cavity dynamics. Results reveal that the conical nose exhibits a self-regulating mechanism, adjusting its effective deadrise angle via angle of attack variations to yield the weakest cavity asymmetry. Conversely, the flat nose experiences strong flow separation; its high sensitivity to trans-media pressure gradients causes the most severe asymmetry. The hemispherical nose lacks both traits, exhibiting asymmetry primarily driven by the free-surface effect. Furthermore, at a 10° exit angle, the low fluid inertia of the thin water layer causes the pitch rate to exceed the velocity deflection rate. This disparity triggers a positive feedback loop involving trajectory deviation and tail slamming. Steeper water-exit angles increase fluid inertia, suppressing this feedback and enabling the vehicle to stably skim the cavity lip. Ultimately, nose shape and exit angle collectively dictate the free-surface effect's intensity. This study elucidates the fundamental mechanisms governing shallow-angle trans-media exits, providing insights into cavity asymmetry, motion characteristics, and hydrodynamic loading.

In this study, the detached eddy simulation (DES) turbulence model coupled with the Schnerr–Sauer cavitation model is employed to numerically investigate the cavity evolution of a rotating underwater vehicle at different rotational speeds and angles of attack. The results reveal the coupled influences of rotation and angle of attack (AOA) on the cavitation flow field characteristics surrounding the vehicle. Specifically, the angle of attack causes the fluid from the upper side of the vehicle to migrate gradually toward the lower side along the body surface, thereby generating two additional streamwise vortices of equal magnitude but opposite direction, leading to a symmetric twin-vortex (TV) structure in the wake. However, the rotation of the vehicle strengthens the vortex with the same rotation direction while weakening that with the opposite direction, thereby resulting in an asymmetric twin-vortex structure, or even a single-vortex structure. Therefore, the internal pressure distribution of the cavity is modified, which subsequently changes the phase transition process between liquid water and vapor. Moreover, cavity evolution analysis shows that the angle of attack predominantly affects the shape of the cavity along its upper and lower surfaces, whereas rotation primarily influences its lateral shape. By modifying the Savchenko empirical formula, a predictive model for the cavity profile under non-zero angle of attack conditions is derived. The analysis demonstrates that the critical wetted angle of attack is inversely proportional to both the cavitation number and the aspect ratio, thereby revealing the constraint relations among aspect ratio, cavitation number, and angle of attack on supercavitating conditions.

A trajectory control method is proposed for the rapid leveling of the projectile after water entry through an asymmetric head-type design. The hydrodynamic and trajectory characteristics of truncated head-type projectiles were investigated using high-speed experiments and numerical simulations based on Reynolds-averaged Navier–Stokes (RANS) equations coupled with the volume of fluid (VOF) method. The effects of truncation angle and water-entry velocity were systematically examined. The results show that all truncated head-type projectiles exhibited rightward-deflected trajectories, with larger truncation angles producing greater lateral deviation and trajectory curvature. This behavior is attributed to asymmetric water impact, where non-uniform flow around the projectile generates distinct cavities at the head and tail, respectively, resulting in lateral forces and yaw moments. The initial impact induced sharp peaks in drag, lateral force, and yaw moment coefficients. As the projectile interacted with the cavity surface, projectiles with larger truncation angles experienced earlier cavity collisions and faster stabilization. In contrast, variations in water-entry velocity had little influence on centroid trajectories and hydrodynamic coefficients at comparable penetration depths. An empirical trajectory model was developed by decoupling the equations of motion to relate trajectory curvature to truncation angle. The model demonstrated excellent agreement with numerical results, providing a reliable predictive tool for the design and control of water-entry projectiles.

When a high-speed projectile navigates underwater near the surface, the unsteady free surface changes the surrounding flow velocity and pressure distribution, which affects the supercavitating flow pattern and hydrodynamic characteristics. Therefore, the three-dimensional underwater supercavitating projectiles near the unsteady surface are numerically investigated using the inflow velocity boundary wave-generation method. The effects of wave height, length, and immersion depth are analyzed, and the coupling mechanism between the unsteady free surface and supercavitating flow is revealed. Under the influence of a wavy surface, the cavity near the surface enlarges and fluctuates opposite to the wave period. Consequently, the supercavity presents an asymmetry that is convex up and flat down. This is because the vapor in the cavity expands, and the water below the cavity is more difficult to displace than the surface connecting the atmosphere, which suppresses the downward expansion of the cavity. Thus, the wave arches upward, and the arching of the trough is stronger than that of the wave crest. The projectile accordingly suffers from an upward viscous lift force, which also fluctuates periodically with waves. As wave heights increase, the fluctuations of cavity shapes and hydrodynamic forces are enhanced. As the immersion depth decreases, the interaction between the water surface and the cavity is augmented. The upper cavity further enlarges and fluctuates more violently, leading to a reduction in drag and an enhancement of lift. When the immersion depth is further reduced, the supercavity connects with the free surface, which results in the shrinkage of the cavity and a significant fluctuation of hydrodynamic forces.