Fish in nature can encounter various flow environments. This paper numerically simulated a 3D (three-dimensional) carangiform fish swimming in oblique flow. The numerical model adopts a robust ghost cell method with graphics processing unit acceleration. The dynamic performance and the 3D wake evolutions are discussed under different Strouhal numbers and attack angles. It is found that the thrust along the swimming direction would get enhanced with more energy consumption as the Strouhal number (St) rises. The attack angle can get the similar but less significant effect. Also, the stall angle of θ = 40° is approximately determined, which is independent of the Strouhal number. However, the flexible deformation can reduce the adverse effects of the stall. In terms of the wake structures, they are transitioned from the two rows of vortex streets at St = 0.2 to the three rows at St = 0.6, and even to the four rows at St = 1. The connected oblique vortex ring rows induced by the undulating caudal fin contributes to the thrust and lateral forces dominantly. As the St rises, the vortex ring rows is transformed from the typical von Karman vortex streets to the reverse one, indicating the generation of thrust. The slender, parallel vortex contrails are caused by the detachment of leading-edge vortices (LEVs), and they induce the high-order harmonic components in force coefficients. The oblique angle of the vortex rings grows with the Strouhal number, while it is hardly affected by the attack angle. As the attack angle grows, the wake is turned from the disconnected hairpin vortices to the intertwined vortex rings and losses the spanwise symmetry. Moreover, the reattachment of the LEV is not observed after the stall angle.

Understanding the water entry in waves is crucial for the structural safety and stability of vehicles. This study numerically investigates the dynamics of the high-speed water entry of a slender, truncated-cone-shaped projectile under varying wave conditions. The numerical model employs the Reynolds-averaged Navier–Stokes method with an overlapping grid. The focus is put on the influence of the wave phases, speeds, and entry angles on the motion trajectory and associated flow field evolution. Wave presence significantly affects cavity formation compared with static water, with pronounced asymmetry observed during vertical water entry at zero-crossing points, leading to asymmetric hydrodynamic forces and pitch deviations. Substantial transient radial forces are generated, raising concerns regarding the structural integrity under adverse sea conditions. At wave crests or troughs, increased wave forces result in higher peak drag forces, particularly during the crest entry. The entry angle significantly influences hydrodynamic performance. A larger entry angle generally increases the drag and alters the lift, thereby affecting the stability and trajectory of the projectile. As the entry angle increased, a greater asymmetry between the left and right cavities appeared, particularly at 25°, where the left cavity was smaller. Moreover, higher entry angles often resulted in increased surface splashing and spray generation. This can affect the aerodynamic characteristics of the projectile and lead to additional drag forces that should be considered in the design. The results can help understand the dynamic loads of vehicles under wave conditions, thereby enhancing their trajectory stability and optimizing the structural design.

This study numerically simulated oblique flows around an undulating fish using a ghost cell method with general processing unit (GPU) acceleration to investigate the hydrodynamic mechanism of swimming fish at an angle to the flow direction. The hydrodynamic performance and wake patterns of swimming fish were assessed under different attack angles (α) and Reynolds numbers (Re) over a wide range of Strouhal numbers (St). The results indicate that a higher St for the nonzero α yields a larger lift-to-drag ratio and thrust, which benefits propulsion and maneuverability. As the α grows, the fish can obtain a higher thrust along the tangential direction with reduced energy consumption. Additionally, the force coefficients shift from the nearly periodic variation to the multiple-period oscillations due to the modulation of several harmonic components. Similarly, increasing Re also enhances the propulsive performance, although to a lesser extent than α and St. Correspondingly, the force pattern becomes increasingly unstable and irregular as the viscous force diminishes. A high St can amplify the effects of α and Re. Diverse wake modes are captured, depending on the α, St, and Re including the von Kármán (VK) vortex modes, the interaction of the leading-edge vortex and trailing-edge vortex, and the multiscale vortex pattern. Thrust is not necessarily induced by the reverse von Kármán (RVK) mode, and it can be generated in both the VK and RVK modes. Furthermore, a higher Re intensifies vortex interactions and wake deflection.

Flow around clustered cylinders is widely encountered in engineering applications such as wind energy systems, pipeline transport, and marine engineering. To investigate the hydrodynamic performance and vortex dynamics of multiple cylinders under forced vibration at low Reynolds numbers, with a focus on understanding the interference characteristics in various configurations, this study is based on a self-developed radial basis function iso-surface ghost cell computing platform, which improves the implicit iso-surface interface representation method to track the moving boundaries of multiple cylinders, and employs a self-constructed CPU/GPU heterogeneous parallel acceleration technique for efficient numerical simulations. This study systematically investigates the interference characteristics of multiple cylinder configurations across various parameter domains, including spacing ratios, geometric arrangements, and oscillation modes. A quantitative analysis of key parameters, such as aerodynamic coefficients, dimensionless frequency characteristics, and vorticity field evolution, is performed. This study reveals that, for a dual-cylinder system, there exists a critical gap ratio between X/D = 2.5 and 3, which leads to an increase in the lift and drag coefficients of both cylinders, a reduction in the vortex shedding periodicity, and a disruption of the wake structure. For a three-cylinder system, the lift and drag coefficients of the two upstream cylinders decrease with increasing spacing. On the other hand, this increased spacing results in a rise in the drag of the downstream cylinder. In the case of a four-cylinder system, the drag coefficients of the cylinders located on either side of the flow direction are relatively high. A significant increase in the lift coefficient occurs when the spacing ratio is less than 2.0, while the drag coefficient of the downstream cylinder is minimized. The findings establish a comprehensive theoretical framework for the optimal configuration design and structural optimization of multicylinder systems, while also providing practical guidelines for engineering applications.

This study presents a gate-assisted flapping-foil system to enhance energy-extraction performance. A two-dimensional (2D) numerical model is established to investigate the energy-extraction performance and efficiency-enhancing mechanism under different flapping frequencies and clearance spacings between the gate and the foil. The intention is to maximize the energy-extraction performance by determining the appropriate gate size and flapping frequency of the foil. The results show that the energy-extraction efficiency exhibits a single peak pattern with the flapping frequency, and there exists an optimal frequency corresponding to the maximum efficiency. The optimal frequency and maximum energy-extraction performance increased with narrowing of the clearance spacing. Compared to the wall-free case, a narrow clearance spacing significantly enhances the energy-extraction efficiency by 116.01%. The lift variation due to the confinement effect is the primary contributor. The two walls promote flow separation and the rolling-up of wall vortices by redirecting and accelerating the vortex flow, leading to the merging and pairing of the vortices. The pressure differences across the foil are amplified, especially when the foil strokes near the wall, which is beneficial for lift generation and energy harvesting. This study enhances the understanding of the energy-extraction mechanism and provides inspiration for innovative energy-harvesting systems.

In confined and intricate aquatic environments, fish frequently encounter the need to propel themselves under oblique flow conditions. This study employs a self-developed ghost-cell immersed boundary method coupled with GPU acceleration technology to numerically simulate the propulsion dynamics of flexible biomimetic fish swimming in oblique flow environments. This research scrutinizes diverse biomimetic fish fin morphologies, with particular emphasis on variations in the Strouhal number and angle of attack, to elucidate hydrodynamic performance and wake evolution. The results demonstrate that as the fin thickness increases, the propulsion efficiency decreases within the Strouhal number range of St = 0.2, 0.4. Conversely, within the range of St = 0.6 to 1.0, the efficiency variations stabilize. For all three fin morphologies, an increase in the Strouhal number significantly augmented both the lift-to-drag ratio and thrust, concomitant with a transition in the wake structure from smaller vortices to a larger alternating vortex shedding pattern. Furthermore, within the Strouhal number range of St = 0.2 to 0.4, the propulsion efficiency exhibits an increase, whereas in the range of St = 0.6 to 1.0, the propulsion efficiency stabilizes. As the angle of attack increases, the drag coefficient increases significantly, while the lift coefficient exhibits a diminishing rate of increase. An increased fin thickness adversely affects the hydrodynamic performance. However, this effect attenuates at higher Strouhal numbers. Conversely, variations in the angle of attack manifest a more pronounced effect on hydrodynamic performance. A thorough investigation and implementation of the hydrodynamic mechanisms demonstrated by swimming fish in complex flow environments enables the development of bio-inspired propulsion systems that not only accurately replicate natural swimming patterns, but also achieve superior locomotion performance and robust environmental adaptability.