The effects of chordwise deformation and the half-amplitude asymmetry on the hydrodynamic performance and vortex dynamics of batoid fish have been numerically investigated, in which the two parameters were represented by the wavenumber ( $W$ ) and the ratio of the half-amplitude above the longitudinal axis to that below ( $HAR$ ). Fin kinematics were prescribed based on biological data. Simulations were conducted using the immersed boundary method. It was found that moderate chordwise deformation enhances the thrust, saves the power and increases the efficiency. A large $HAR$ can also increase thrust performance. By using the derivative-moment transformation theory at several subdomains to capture the local vortical structures and a force decomposition, it was shown that, at high Strouhal numbers ( $St$ ), the tip vortex is the main source of thrust, whereas the leading-edge vortex (LEV) and trailing-edge vortex weaken the thrust generation. However, at lower $St$ , the LEV would enhance the thrust. The least deformation ( $W=0$ ) leads to the largest effective angle of attack, and thus the strongest vortices. However, moderate deformation ( $W=0.4$ ) has an optimal balance between the performance enhancement and the opposite effect of different local structures. The performance enhancement of $HAR$ was also due to the increase of the vortical contributions. This work provides a new insight into the role of vortices and the force enhancement mechanism in aquatic swimming.

In this study, numerical simulations are employed to investigate the hydrodynamic performance and wake topology of a swimming Rhinoptera javanica. The study is motivated by the quest to understand the hydrodynamics of median and paired fin(MPF) mode with both spanwise and chordwise flexibility. The simulations employ an immersed boundary(IB)-simplified sphere function-based gas kinetic scheme(SGKS) method that allows us to simulate flows with complex moving boundaries on fixed Cartesian grids. A computational model is constructed based on biological data. The evolution of the hydrodynamic force and 3D vortex structures are presented. Besides, the effect of frequency and amplitude are also discussed to explain some behaviors of the actual Rhinoptera javanica. This work can provide a baseline for the design of a bio-inspired underwater vehicle.

Flow around a tethered model of a swimming batoid fish is studied by using the wall-modelled large-eddy simulation in conjunction with the immersed boundary method. A Reynolds number ( $Re$ ) up to 148 000 is chosen, and it is comparable to that of a medium-sized aquatic animal in cruising swimming state. At such a high $Re$ , we provide, to the best of our knowledge, the first evidence of hairpin vortical (HV) structures near the body surface using three-dimensional high-fidelity flow field data. It is observed that such small-scale vortical structures are mainly formed through two mechanisms: the leading-edge vortex (LEV)–secondary filament–HV and LEV–HV transformations in different regions. The HVs create strong fluctuations in the pressure distribution and frequency spectrum. Simulations are also conducted at $Re=1480$ and 14 800 to reveal the effect of Reynolds number. Variations of the flow separation behaviour and local pressure with $Re$ are presented. Our results indicate that low- $Re$ simulations are meaningful when the focus is on the force variation tendency, whereas high- $Re$ simulations are needed when concerning flow fluctuations and turbulence mechanisms.

The optimization of the water-entry strategy for cross-wing underwater vehicles has become a research hotspot in the field of engineering, and its water-entry process is quite different from that of wedges and cylinders. In order to address this problem, a water-entry numerical model for the cross-wing underwater vehicle was first established based on the CFD method. The governing equations and boundary conditions of the dynamic model were defined, along with the basic principles of discretization and turbulent flow of the governing equations. The overset mesh and the VOF multiphase flow model were introduced, and a mesh size independence analysis of the numerical model was conducted. Furthermore, the numerical results were compared with the experimental results to ensure the accuracy of the numerical model. The research focused on the cross-wing underwater vehicle’s impact with calm water and regular waves, respectively. The results show that: (1) the numerical simulations are in good agreement with the experimental results (the maximum predictive error is less than 10%), which verifies the accuracy of the numerical model in this paper; (2) when the cross-wing underwater vehicle impacts calm water, the slamming pressure curve firstly shows a trend of increasing, reaching a peak, and then decreasing sharply, and finally stabilizes. As the water-entry velocity increases, the peak slamming pressure exhibits a gradual increase; (3) during the water entry of the cross-wing underwater vehicle into calm water, the acceleration profile demonstrates a trend of initial increase, followed by a decrease, another increase, and then a subsequent decrease as the entry velocity continues to rise. It should be noted that there are two peaks in the acceleration, with the first peak being significantly smaller than that of the second; (4) when the cross-wing underwater vehicle impacts a regular wave, the slamming pressure is lowest when impacting the crest and highest when impacting the trough.

Self-propelled models are widely used to study fish-like swimming, yet most existing studies rely on two-dimensional simplifications and restricted degrees of freedom in the self-propelled motion. In this study, we systematically investigate how dimensionality and kinematic freedom influence the hydrodynamic performance of self-propelled swimmers by varying the undulation frequency, tailbeat amplitude, and fluid viscosity. The results show that two-dimensional models overestimate swimming speed, while increasing the rotational degrees of freedom reduces the swimming speed in both two- and three-dimensional cases. Generally, higher swimming speeds are consistently associated with higher power expenditure. Based on these results, we identify a data-driven parameter that can map two-dimensional results to their three-dimensional counterparts and clarify the limitations of such simplifications. These findings provide insight into the hydrodynamic performance of swimming fish and offer potential guidance for improving reduced-order models of fish-like locomotion.

Seafloor landing research is crucial for underwater robots tasked with long-term fixed-point observation missions, particularly those requiring prolonged operations on the seabed. Developing safe and stable landing technology is essential for the success of these missions. This paper introduces a novel underwater robot by establishing its dynamic model and using the arbitrary Lagrangian–Eulerian (ALE) method to conduct a comprehensive study of its deep-sea landing process. Prior to detailed parameter analysis, the ALE algorithm’s effectiveness and accuracy were validated through experimental data and simulation results from previous studies. This study examines the penetration depth and force conditions during the landing process by varying factors such as seabed soil properties, landing velocity, and the robot’s mass. Findings indicate that seabed soil properties significantly influence penetration depth and pressure, with variations in soil density and strength affecting the robot’s landing behavior. In contrast, the robot’s mass has a relatively minor effect, suggesting that the choice of structural materials has limited impact on penetration depth and pressure during landing. Additionally, landing velocity was found to significantly affect penetration depth and pressure; higher velocities result in greater penetration depths and pressures. This highlights the importance of controlling landing velocity to minimize impact forces and protect the robot. The results of this study provide theoretical support and data for developing deep-sea landing technology for novel underwater robots and inform the selection of structural materials, ensuring the successful execution of long-term fixed-point observation missions in deep-sea environments.