Search Results (205)

Biomimetic pectoral fin propulsion offers a low-noise, highly maneuverable alternative to conventional propellers for next-generation underwater robotic systems. This study develops a manta ray-inspired dual-servo pectoral fin module with a CPG-based controller and employs it as a single-fin test article in a recirculating water tunnel to quantify its hydrodynamic performance. Controlled experiments demonstrate that the fin generates stable thrust over a range of flapping amplitudes, with mean thrust increasing markedly as the amplitude rises, while also revealing an optimal frequency band in which thrust and thrust work are maximized and beyond which efficiency saturates. To interpret these trends, a quasi-steady CFD analysis using the k–ω SST turbulence model is conducted for a series of static angles of attack representative of the instantaneous effective angles experienced during flapping. The simulations show a transition from attached flow with favorable lift-to-drag ratios at moderate angles of attack to massive separation, deep stall, and high drag at extreme angles, corresponding to high-amplitude fin motion. By linking the experimentally observed thrust saturation to the onset of deep stall in the numerical flow fields, this work establishes a unified experimental–numerical framework that clarifies the hydrodynamic limits of pectoral fin propulsion and provides guidance for the design and operation of low-noise, highly maneuverable biomimetic underwater robots. Full article

Background: This study aimed to explore the effects of neuromuscular electrical stimulation (NMES) combined with water-based resistance training on muscle activation and coordination during freestyle kicking. Methods: Thirty National Level male freestyle swimmers were randomly assigned to an experimental group (NMES + water-based training) or a control group (water-based training only) for a 12-week intervention. The experimental group received NMES pretreatment before each session. Underwater surface electromyography (sEMG) synchronized with high-speed video was used to collect muscle activation data and corresponding kinematic information during the freestyle kick. The sEMG signals were then processed using time-domain analysis, including integrated electromyography (iEMG), which reflects the cumulative electrical activity of muscles, and root mean square amplitude (RMS), which indicates the intensity of muscle activation. Non-negative matrix factorization (NMF) was further applied to extract and characterize muscle synergy patterns. Results: The experimental group showed significantly higher iEMG and RMS values in key muscles during both kicking phases. Within the core propulsion synergy, muscle weighting of vastus medialis and biceps femoris increased significantly, while activation duration of the postural adjustment synergy was shortened. The number of synergies showed no significant difference. Conclusions: NMES combined with water-based resistance training enhances muscle activation and optimizes neuromuscular coordination strategies, offering a novel approach to improving sport-specific performance. Full article

Background: Neuromuscular electrical stimulation (NMES) is an effective exogenous neuromuscular activation method widely used in sports training and rehabilitation. However, existing research primarily focuses on land-based sports or single-joint movements, with limited in-depth exploration of its intervention effects and underlying neuromuscular control mechanisms for complex, multi-joint coordinated aquatic activities like breaststroke swimming. This study aimed to investigate the effects of NMES combined with traditional resistance training on neuromuscular function during sport-specific technical movements in breaststroke athletes. Methods: A randomized controlled trial was conducted with 30 national-level or above breaststroke athletes assigned to either an experimental group (NMES combined with traditional squat resistance training) or a control group (traditional squat resistance training only) for an 8-week intervention. A specialized fully waterproof wireless electromyography (EMG) sensor system (Mini Wave Infinity Waterproof) was used to synchronously collect surface EMG signals from 10 lower limb and trunk muscles during actual swimming, combined with high-speed video for movement phase segmentation. Changes in lower limb explosive power were assessed using a force plate. Non-negative matrix factorization (NMF) muscle synergy analysis was employed to compare changes in muscle activation levels (iEMG, RMS) and synergy patterns (spatial structure, temporal activation coefficients) across different phases of the breaststroke kick before and after the intervention. Results: Compared to the control group, the experimental group demonstrated significantly greater improvements in single-leg jump height (Δ = 0.06 m vs. 0.03 m) and double-leg jump height (Δ = 0.07 m vs. 0.03 m). Time-domain EMG analysis revealed that the experimental group showed more significant increases in iEMG values for the adductor longus, adductor magnus, and gastrocnemius lateralis during the leg-retraction and leg-flipping phases (p < 0.05). During the pedal-clamp phase, the experimental group exhibited significantly reduced activation of the tibialis anterior alongside enhanced activation of the gastrocnemius. Muscle synergy analysis indicated that post-intervention, the experimental group showed a significant increase in the weighting of the vastus medialis and biceps femoris within synergy module 4 (SYN4, related to propulsion and posture) (p < 0.05), a significant increase in rectus abdominis weighting within synergy module 3 (SYN3, p = 0.033), and a significant shortening of the activation duration of synergy module 2 (SYN2, p = 0.007). Conclusions: NMES combined with traditional resistance training significantly enhances land-based explosive power in breaststroke athletes and specifically optimizes neuromuscular control strategies during the underwater breaststroke kick. This optimization is characterized by improved activation efficiency of key muscle groups, more economical coordination of antagonist muscles, and adaptive remodeling of inter-muscle synergy patterns in specific movement phases. This study provides novel evidence supporting the application of NMES in swimming-specific strength training, spanning from macroscopic performance to microscopic neural control. Full article

Inspired by the movements of sea turtle forelimbs, this study presents a bio-inspired underwater flapping wing with three degrees of freedom. This flapping wing mechanism can more accurately simulate the rotational motion of a sea turtle’s forelimbs to generate greater propulsive force. The highlight is the gear transmission mechanism arranged along the wingspan, enabling a preset increasing twist angle along the wingspan. Computational fluid dynamics simulations are conducted to evaluate the hydrodynamic performance of the proposed flapping wing system. The effects of different spanwise twist angles along the wingspan on thrust generation are quantitatively analyzed, as well as the influence of key kinematic parameters, including the longitudinal flapping angle, spanwise increasing twist angle, and elevation angle. The results indicate that, compared with a uniform twist angle, the spanwise increasing twist significantly increases the peak thrust during specific phases of the flapping cycle. It is further revealed by flow field analyses that the formation of vortices near the trailing edge enhances the propulsive force in the streamwise direction. To further validate the proposed concept, a prototype of the mechanism is fabricated and experimentally tested under low-frequency actuation, confirming the feasibility of the mechanical design. Overall, these results demonstrate the potential of the proposed approach for bio-inspired underwater propulsion and provide useful guidance for future flapping wing mechanisms and kinematic design. Full article

The evolution of modern warfare and civil exploration requires platforms that can operate seamlessly across the air–water interface. The folding-wing Hybrid Air and Underwater Vehicle (FUD) has emerged as a transformative solution, combining the high-speed cruising capabilities of fixed-wing aircraft with the stealth characteristics of underwater navigation. This review thoroughly analyzes the advancements and challenges in folding-wing FUD technology. The discussion is framed around four interconnected pillars: the overall design driven by morphing technology, adaptation of the propulsion system, multi-phase dynamic modeling and control, and experimental verification. The paper systematically compares existing technical pathways, including lateral and longitudinal folding mechanisms, as well as dual-use and hybrid propulsion strategies. The analysis indicates that, although significant progress has been made with prototypes demonstrating the ability to transition between air and water, core challenges persist. These challenges include underwater endurance, structural reliability under impact loads, and effective integration of the power system. Additionally, this paper explores promising application scenarios in both military and civilian domains, discussing future development trends that focus on intelligence, integration, and clustering. This review not only consolidates the current state of technology but also emphasizes the necessity for interdisciplinary approaches. By combining advanced materials, computational intelligence, and robust control systems, we can overcome existing barriers to progress. In conclusion, FUD technology is moving from conceptual validation to practical engineering applications, positioning itself to become a crucial asset in future cross-domain operations. Full article

Maintenance operations represent one of the most underutilized opportunities to reduce emissions and improve the energy efficiency of ships. This study proposes an innovative approach that analyzes such interventions from a holistic perspective of energy, environment, and economics using real operational data from two liquefied natural gas (LNG) carriers before and after their maintenance operations. The results show that comprehensive actions such as complete hull and propeller cleaning can reduce fuel consumption by more than 30% and CO₂ emissions by more than 15%, in addition to improving propulsive efficiency by between 18% and 34%. In contrast, minor interventions, such as underwater propeller cleaning, have a limited effect with very specific improvements in fuel savings at certain speed ranges, but no significant effect on emissions or shaft power. In particular, the study demonstrates that a single comprehensive maintenance operation can change the Carbon Intensity Indicator (CII) rating from category E to D, reinforcing the strategic role of maintenance in the decarbonization and revaluation of maritime transport. Full article

To enhance the flexibility and adaptability of underwater robots in complex environments, this paper designs an octopus-inspired soft underwater robot capable of both bipedal walking and multi-arm swimming. The robot features a rigid–flexible coupling structure consisting of a head module and eight rope-driven soft tentacles and integrates buoyancy adjustment and center-of-gravity balancing systems to achieve stable posture control in both motion modes. Based on the octopus’s bipedal walking and multi-arm swimming mechanisms, this study formulates gait generation strategies for each mode. In walking mode, the robot achieves underwater linear movement, turning, and in-place rotation through coordinated tentacle actuation; in swimming mode, flexible three-dimensional propulsion is realized via synchronous undulatory gaits. Experimental results demonstrate the robot’s peak thrust of 14.1 N, average swimming speed of 8.6 cm/s, and maximum speed of 15.1 cm/s, validating the effectiveness of the proposed structure and motion control strategies. This research platform offers a promising solution for adaptive movement and exploration in unstructured underwater environments. Full article

Wind power generation ships (WPG ships), which combine rigid sails for propulsion and underwater turbines for onboard power generation, have attracted increasing attention as a promising concept for utilizing renewable energy at sea. This study presents an integrated assessment of a WPG ship by combining towing-tank experiments, CFD simulations using ANSYS Fluent, and theoretical analysis to evaluate the coupled performance of sails, hull, and underwater turbines. First, sail thrust and bare-hull resistance were quantified to identify the effective operating-speed range under Beaufort 6–8 wind conditions, and the optimal number of rigid sails was determined. Based on a thrust–resistance balance at a representative rated operating point, two turbine configurations (two and four turbines) were preliminarily sized. The results show that ten rigid sails can provide near-maximum thrust without excessive aerodynamic interference, and the installation of turbines significantly reduces the feasible operating range compared to the bare-hull case. For the two-turbine configuration, a common effective ship-speed range of 6.58–8.0 m/s is obtained, whereas the four-turbine configuration is restricted to 6.58–7.44 m/s due to wake losses, additional appendage drag, and near-free-surface effects. The four-turbine configuration exhibits approximately 30% lower total power output than the two-turbine configuration. These findings demonstrate that an integrated, system-level evaluation is essential for WPG ship design and indicate that the two-turbine configuration offers a more favorable balance between power generation capability and operational flexibility. Full article

To overcome the limitations in maneuverability and adaptability of traditional underwater vehicles, a novel hybrid-actuated, multimodal cephalopod-inspired robot is proposed. This robot innovatively integrates a hybrid drive system wherein sinusoidal undulating fins provide primary propulsion and steering, water-flapping tentacles offer auxiliary burst propulsion, and a gear-and-rack center-of-gravity (CoG) adjustment module modulates the pitch angle to enable depth control through hydrodynamic lift during forward motion. The effectiveness of the design was validated through a series of experiments. Thrust tests demonstrated that the undulating fin thrust scales quadratically with oscillation frequency, aligning with hydrodynamic theory. Mobility experiments confirmed the multi-degree-of-freedom control of the robot, demonstrating effective diving and surfacing via the CoG module and high maneuverability, achieving a turning radius of approximately 15 cm through differential fin control. Furthermore, field trials in an outdoor artificial lake with a depth of less than 1 m validated its environmental robustness. These results confirm the versatile maneuvering capabilities of the robot and its robust adaptability to confined and shallow-water environments, presenting a novel platform for complex underwater observation tasks. Full article

Unmanned underwater vehicles (UUVs) capable of agile, high-speed maneuvering in complex environments require propulsion systems that can dynamically modulate three-dimensional forces. The California sea lion (Zalophus californianus) provides an exceptional biological model, using its foreflippers to achieve rapid turns and powerful propulsion. However, the specific kinematic mechanisms that govern instantaneous force generation from its powerful foreflippers remain poorly quantified. This study experimentally characterizes the time-varying thrust and lift produced by a bio-robotic sea lion foreflipper to determine how flipper twist, sweep, and phase overlap modulate propulsive forces. A three-degree-of-freedom bio-robotic flipper with a simplified, low-aspect-ratio planform and single compliant hinge was tested in a circulating flow tank, executing parameterized power and paddle strokes in both isolated and combined-phase trials. The time-resolved force data reveal that the propulsive stroke functions as a tunable hybrid system. The power phase acts as a force-vectoring mechanism, where the flipper’s twist angle reorients the resultant vector: thrust is maximized in a broad, robust range peaking near 45°, while lift increases monotonically to 90°. The paddle phase operates as a flow-insensitive, geometrically driven thruster, where twist angle (0° optimal) regulates thrust by altering the presented surface area. In the full stroke, a temporal-phase overlap governs thrust augmentation, while the power-phase twist provides robust steering control. Within the tested inertial flow regime (Re ≈ 10⁴–10⁵), this control map is highly consistent with propulsion dominated by geometric momentum redirection and impulse timing, rather than circulation-based lift. These findings establish a practical, experimentally derived control map linking kinematic inputs to propulsive force vectors, providing a foundation for the design and control of agile, bio-inspired underwater vehicles. Full article

A parallel-type vector thruster was designed, and its dynamic performance was systematically investigated using a combination of mechanical structural analysis, numerical calculations, and experimental methods. First, a thrust model of the vector thruster was established, and its mechanical structural characteristics were analyzed. Subsequently, the governing equations were discretized and solved using computational fluid dynamics (CFD), and a numerical model of the thruster’s viscous flow field at different inclination angles was established based on the moving reference frame (MRF) method. Furthermore, the structural forces, wake effects, and hydrodynamic performance of the thruster under different rotational speeds and inclination angles were analyzed using the control variable method. Finally, the thrust performance was tested using an underwater dynamic test rig, and the experimental results were compared and analyzed with the theoretical calculations. The results suggest that as the propeller’s inclination angle increases, the low-pressure region on the thruster tends to expand, which appears to intensify cavitation and vortex phenomena and leads to a more uneven wake distribution. These effects are correlated with a reduction in propulsion efficiency, which was observed to increase nonlinearly with rotational speed, potentially influencing the thruster’s operational performance. The study elucidates the influence mechanism of steering mechanism stiffness on the propulsion characteristics of spatially moving propellers, providing a foundation for future model-based control design in vector thrust systems. Full article

The locomotion of fish provides inspiration for designing efficient and agile underwater robots. Potamotrygon motoro propels itself by generating traveling waves along its pectoral fins. Inspired by its graceful swimming stroke, we design and fabricate a robotic fish, where the snap-through instability of elastic curved rods is exploited to produce the undulatory fin motion. In this design, the rotary input of two motors is transformed smoothly and continuously to controllable wave-like fin deformation. By changing the initial fin shape, motor speed, and friction at the releasing end, the propulsion performance and the maneuverability of the robotic fish can be significantly improved. The physical prototype of the robotic fish is fabricated, and its swimming performance is measured. Its maximum swimming speed reaches 0.76 BL/s, and it can achieve small-radius turns with a maximum angular speed of 1.25 rad/s. In contrast to the multi-actuator systems, the proposed dual-motor, elastic-snapping–driven design is featured by simple structural construction, low energy consumption, excellent maneuverability, and superb adaptation to environments. Our robotic fish holds promising applications in such areas as environmental monitoring, underwater inspection, and ocean exploration. The propulsion strategy presented in this work may pave a new way for the design of shape-morphing robots as well as other soft machines at multiple length scales. Full article

To address the inherent complexities of underwater operating environments and achieve the design of a highly efficient, energy-saving flapping hydrofoil, this paper proposes an intelligent agent-based model for real-time parametric optimization. A non-parametric surrogate model based on a Multilayer Perceptron (MLP) is established using data samples of multi-dimensional flapping hydrofoil geometric parameters obtained through Computational Fluid Dynamics (CFD) simulations. An improved Double Deep Q-Network (DDQN) algorithm incorporating Pareto frontier information is deployed within the surrogate model to obtain the Pareto optimal solution set for propulsion efficiency and average input power, and a set of propulsion parameter combinations with error ranges between 0.24% and 1.27% across continuous intervals was obtained. Experimental results demonstrate that the proposed MLP-DDQN method is capable of learning the domain-wide optimal solution within the experimental environment, satisfying the Pareto optimality between propulsion efficiency and average input power. Further analysis of the flow field around the flapping hydrofoil under the obtained optimal parameter combination revealed that the presence of stable and continuously attached vortex structures on the wing surface is the intrinsic mechanism responsible for its superior propulsion performance. Full article

To address the challenge of head stability in a biomimetic robotic dolphin during self-propulsion, this study systematically investigates the passive stabilization mechanism of a bio-inspired caudal keel. A combined experimental and computational fluid dynamics (CFD) approach was employed to evaluate four keel geometries across a tail oscillation frequency range of 0.5–2 Hz. The experimental results demonstrate that the optimal keel configuration reduced the standard deviation of the head pitch angle by 20.9% at 2 Hz. CFD analysis revealed a dual stabilization mechanism: an effective keel not only attenuates the intensity of the primary disturbance moment at the driving frequency but, more critically, also enhances the spectral purity of the signal by suppressing high-frequency harmonics and broadband stochastic noise through the systematic reorganization of caudal vortices. A systematic investigation of keel geometry identified non-dimensional height (h/c) as the dominant parameter, with its stabilizing effect exhibiting diminishing returns beyond an optimal range. Furthermore, a quantifiable design trade-off was established, showing an approximate 9.1% increase in the Cost of Transport (CoT) for the most stable configuration. These findings provide quantitative design principles and a deeper physical insight into the passive stabilization of biomimetic underwater vehicles, highlighting the importance of both disturbance intensity and spectral quality. Full article

Unmanned aerial underwater vehicles (UAUVs) will play significant roles in several complex application scenarios including observation of mesoscale ocean phenomena, monitoring of offshore platforms, ocean protection, and maritime rescue. These innovative vehicles can be used in the air and underwater and can easily enter and exit water. This review systematically analyzes the research progress, design challenges, and future prospects of UAUVs, emphasizing their potential to revolutionize integrated cross-domain collaboration. We classify UAUVs into five categories—rotary-wing, fixed-wing, folding-wing, hybrid-wing, and flapping-wing—based on propulsion configurations, and critically evaluate their prototypes, highlighting technological milestones and functional limitations. Unlike prior reviews focused solely on technical developments, this study advocates for a paradigm shift from a technology-push to a market-pull and technology-push interactive development model. Combining the design of UAUV with solutions to technical challenges and specific application requirements is crucial for practical deployment. By synthesizing historical context, current advancements, and future developments, this review not only provides possible strategies for design challenges but also lays a roadmap for UAUV commercialization. Full article