To address the difficulty of reconciling propulsion capability with high maneuverability in underwater robot propulsion systems, a coaxial propulsion system with a cycloidal-helical twin-screw configuration is proposed. The hydrodynamic characteristics of the system are investigated using the dynamic mesh method in ANSYS Fluent, with a focus on the axial propulsion response under different axial spacings. Taking a single propeller as the baseline, the axial propulsion performance and flow field characteristics of the coaxial system are comparatively analyzed under different axial spacings. The results show that the axial propulsion performance exhibits a pronounced non-monotonic dependence on spacing. The optimal performance is achieved at a spacing of 0.8D, where the average axial thrust increases by 42.83% compared with the single propeller. A moderate improvement is observed at 0.7D, whereas no significant advantage is found at 0.6D, 0.9D, or 1.0D. Further analysis indicates that the performance enhancement mainly originates from the modulation effect of the upstream cycloidal propeller on the inflow of the downstream propeller, rather than the direct axial thrust contribution of the cycloidal propeller itself. The results provide a theoretical reference for the design and optimization of coaxial propulsion systems for high-maneuverability underwater robots.

As a novel propulsion device, the cycloidal propeller offers superior maneuverability and controllability. However, there is still insufficient understanding of how cycloidal propellers should be arranged on a vehicle so that their advantages can be fully exploited, enabling more rational thrust distribution and improved control of the pressure and vorticity fields, and thereby optimizing hydrodynamic performance. To clarify the fluid-dynamic principles underlying this problem, this study takes a four-cycloidal-propelled underwater vehicle as the research object and systematically investigates the planar motion performance of two propulsor layouts, namely the H-configuration and the X-configuration, using computational fluid dynamics. First, the two vehicle configurations and their corresponding planar motion modes are established. Then, the hydrodynamic parameters, pressure distributions, and three-dimensional vortex structures of the two configurations during planar motion are comparatively analyzed. Finally, the performance differences caused by the configuration change are interpreted from the perspectives of thrust distribution, wake interaction, and flow-field reconstruction. The results show that the H-configuration provides a higher degree of thrust organization along the motion direction, thereby producing more coherent wakes, reducing inter-propulsor interference, and achieving higher steady cruising speed and propulsion efficiency. In contrast, in the X-configuration, part of the propulsive effort generates lateral force components and is canceled during force synthesis; meanwhile, stronger wake interaction and broader vortex spreading further weaken the effective propulsion, resulting in lower planar-motion speed and efficiency. However, by utilizing the lateral force components generated by the propulsors, the X-configuration can achieve omnidirectional planar motion without changing the body orientation. This study explains, from a fluid-dynamics perspective, the intrinsic mechanism by which propulsor layout determines the planar motion performance of cycloidal-propelled underwater vehicles and can provide a reference for future research on propulsion layout design.

To address the issues of high disturbance, fixed configuration, and poor environmental adaptability in conventional underwater robots, this paper proposes a modular underwater robot with undulating fin propulsion, integrating biomimetic propulsion with modular reconfigurable concepts. Each individual module of the robot possesses autonomous underwater propulsion capabilities. Focusing on the individual module, the mechanical structure design and control system development were completed, and an experimental platform was established to conduct propulsion performance experiments. Specifically, variable parameter experiments were performed to evaluate the tethered thrust and average linear propulsion velocity of the individual module. The experimental results demonstrate that increasing both frequency and amplitude can enhance the propulsion performance of the module. For the π/8 amplitude condition, the enhancement effect of amplitude on propulsion performance is superior to that of a simple frequency increase. Furthermore, when the frequency exceeds 1.4 Hz, the promotional effect of amplitude on propulsion performance gradually diminishes. The research findings confirm that the designed individual module meets the basic propulsion performance requirements and establishes a foundation for the subsequent development and testing of multi-module reconfigurable systems.

To investigate planar omnidirectional locomotion of a four-module undulating-fin underwater robot, this study considers a square configuration that forms two orthogonal fin groups. A planar omnidirectional locomotion strategy based on the vector synthesis of two orthogonal undulating-fin propulsion components is proposed and examined using self-propelled computational fluid dynamics simulations. The results show that the motion direction is mainly determined by the relative magnitudes of the propulsion components generated by the Fin-X and Fin-Y undulating-fin groups. By regulating the frequency or amplitude of the two fin groups, the robot can continuously adjust its direction of motion within the horizontal plane. Analyses of the pressure field, vorticity field, and three-dimensional vortex structures indicate that propulsion by a single fin group produces a strongly directional wake. When the two fin groups have identical parameters, the robot exhibits better attitude stability, whereas parameter mismatch strengthens multi-fin flow-field coupling and leads to more pronounced transient fluctuations. Frequency regulation primarily affects the direction of motion by changing the driving period and the temporal coupling of the flow field, while maintaining good attitude stability. Amplitude regulation influences propulsion capability by changing the structural motion amplitude and the intensity of fluid disturbance, and shows advantages in propulsive efficiency. This study clarifies the hydrodynamic characteristics and regulation mechanisms underlying planar omnidirectional locomotion in a four-module square-configuration undulating-fin robot, providing a basis for configuration design and highly maneuverable motion control of modular biomimetic underwater robots.

The propulsion performance and maneuverability of underwater vehicles are often difficult to achieve simultaneously. For example, while cycloidal underwater vehicles possess excellent maneuverability, their propulsion performances are relatively limited. To address this limitation, this study designs an underwater vehicle equipped with two cycloidal propellers and explores the use of leading-edge and trailing-edge double-flap blades to enhance propulsion performance. First, the feasibility of improving the virtual camber effect of the cycloidal propeller by adding double flaps is analyzed from a theoretical perspective. Second, a prototype underwater vehicle equipped with two cycloidal propellers is developed, and experiments are conducted to verify its ability to perform predetermined maneuvers such as forward motion, lateral motion, and in situ turning. Finally, the double-flap blades are integrated into the cycloidal-propelled prototype, and computational fluid dynamics simulations are performed to analyze the hydrodynamic performance of no-flap and double-flap configurations under different operating modes, thereby verifying the propulsion enhancements introduced by the flaps and elucidating the underlying mechanisms. The results show that the cycloidal underwater vehicle equipped with leading-edge and trailing-edge double-flap blades achieves up to a 72.3% increase in forward motion propulsion speed compared with the no-flap configuration, while effectively reducing speed fluctuations. In lateral motion mode, the increased hydrodynamic drag results in only a 9.9% improvement in maximum speed. In in situ turning mode, the turning speed of the vehicle increases by 26.6%. Moreover, in forward motion, lateral motion, and in situ turning modes, the thrust coefficient and torque coefficient of the double flaps underwater vehicle increase significantly compared with the no-flap configuration, thereby achieving the goal of enhancing propulsion performance without compromising maneuverability. This study proposes and validates a method for improving the propulsion performance of cycloidal underwater vehicles, providing scientific support for the development of next-generation prototype autonomous underwater vehicles.

To enhance the surface operation capabilities of traditional bionic underwater vehicles (BUVs) in, this study explored the feasibility of dolphins performing cross-medium standing-and-turning (SAT) behavior on the water surface from a hydrodynamics perspective. A physical model and computational model of the robotic dolphin’s surface SAT behavior were established. After numerous attempts, the surface SAT behavior of the robotic dolphin was successfully replicated through coordinated movements of the body, caudal fin, and pectoral fins, and the quantitative relationship between controllable parameters and hydrodynamic performance was investigated. By combining data analysis and flow field distribution patterns, the underlying physical mechanisms of the robotic dolphin’s surface SAT behavior were revealed. The results indicate that the turning trajectory of SAT behavior exhibits a circular characteristic, and the turning radius can be adjusted by modifying the kinematic parameters. Additionally, when the movement parameters of the body and caudal fin are fixed, and the phase difference between the two pectoral fins is T/2, the robotic dolphin achieves optimal turning maneuverability, with a maximum turning speed of 1.69 rad/s and a turning efficiency of up to 45.5%. Notably, by optimizing kinematic parameters, the robotic dolphin achieves cross-medium in-situ turning with exceptionally high maneuverability, which is indeed a very valuable discovery. The findings provide a cross-medium fluid dynamics explanation for the development of BUVs with dual underwater/surface operating capabilities.

To provide a hydrodynamic basis for improving the adaptability and maneuverability of underwater robots in complex environments, a modular underwater robot driven by biomimetic undulating fins is proposed. A four-module rectangular configuration composed of front, rear, left, and right propulsion units is considered, and its self-propulsion hydrodynamic characteristics are systematically investigated using computational fluid dynamics simulations. The effects of uniform fin actuation on propulsion performance are analyzed by varying fin undulation frequency and amplitude. The results show that, under different parameter combinations, the rear fins consistently generate stronger propulsion contributions than the front fins, indicating pronounced asymmetry in multi-fin hydrodynamic interactions. Motivated by this phenomenon, differential front–rear fin actuation cases with different fin frequencies and amplitudes are further investigated. The results indicate that frequency differences exert a stronger influence on propulsion stability and transient force fluctuations, whereas amplitude differences mainly affect thrust magnitude with relatively weaker influence on dynamic stability. Furthermore, exchanging the front–rear fin parameters demonstrates that configurations with smaller front fin parameters and larger rear fin parameters achieve superior propulsion performance. The present study reveals how both uniform and front–rear differential fin actuation influence vortex interaction, momentum redistribution, and coupled wake dynamics in multi-fin propulsion systems. These findings provide theoretical guidance for the hydrodynamic design and control of modular self-reconfigurable underwater robots, while also contributing to the understanding of unsteady propulsion physics in undulating fin systems.

For oscillating-body wave energy converters (WECs), converting the kinetic energy of the floater's multi-degree-of-motion (multi-DOF) motions to electricity using a single power take-off (PTO) system may be attracting due to high power capture efficiency and reliability. In this work, a novel PTO system is proposed to gather the floater's two-DOF motions, i.e. surge and heave, using a single output shaft via a total of four rope transmission systems. Since the output shaft is simultaneously driven by two-independent-DOF motions, the PTO system is over-actuated. It is found that, when the surge restoring coefficient is rather small or large, the floater's surge velocity is too low to catch up with the output shaft, hence only the heave motion of the floater contributes to wave energy harvesting. Otherwise, both surge and heave motions engage in wave energy harvesting, and the power capture efficiency peaks when the surge motion is brought into resonance. Compared to traditional PTO systems that each can only extract the kinetic energy of the floater's single-DOF motion, this over-actuated PTO system can significantly improve power capture performance, demonstrated by the maximum capture width ratio exceeding the theoretical limit of either DOF.

In order to improve the operational capability of underwater robots in different environments, a variable configuration robot with multiple undulating fins collaboration is proposed to realize the different operational requirements of the same individual robot. In this study, a variable configuration robot with multiple undulating fins collaboration was designed using the flexible undulating fins of the Gymnarchus niloticus fish as a bionic object. It consists of a body and four undulating fin modules that can be transformed between H-configuration and X-configuration according to the operational requirements. The hydrodynamic characteristics of the two configurations during self-propulsion were studied using numerical simulations. The wavenumber, frequency, and amplitude of the undulating fins were used as controllable parameters to investigate the effects on the self-propulsion performance of the robot and each undulating fin module. The evolution laws of the vortex structure in the self-propulsion of the two configurations were comparatively analyzed. The results show that the trends of the velocity coefficient, propulsion coefficient, and propulsion efficiency with controllable parameters in the steady-state propulsion phase are largely the same for both configurations. When the robot is in H-configuration, the interaction between the anterior and posterior fins on the same side increases the propulsive force coefficient of the posterior fin; when the robot is in X-configuration, the four fins have less interaction with each other, and their propulsive force coefficients are similar. Further comparisons revealed that the robot self-propelled faster and propelled more efficiently in the H-configuration, while providing better stability in the X-configuration.

Considering the advantages of biomimetic undulating fin propulsion, a modular underwater robot was developed, and the linear underwater propulsion performance of its individual module was studied. The CFD method was used to analyze the hydrodynamic characteristics of the robot's underwater self-propulsion, verifying the feasibility of the modular robot's self-propulsion and the overall design's rationality. The variation in speed and thrust of the robot over time during its underwater self-propulsion process was clarified. The transient evolution of the undulating fin, robot and surrounding flow field during the steady-state propulsion cycle was analyzed. A parametric study was conducted to examine the effect of different undulating parameters of the fin on the robot's underwater propulsion performance. The effect of frequency on propulsion performance under different amplitude conditions showed a certain similarity. The effect of amplitude on propulsion performance under different frequency conditions followed the same trend, and when the frequency exceeded 2 Hz, the curve of propulsion performance with respect to amplitude became more similar. The research results provide a theoretical fluid dynamics foundation for undulating fin robot underwater propulsion and also serve as the basis for future research on underwater propulsion in modular reconfigured forms.

An invert mode for a multi-tentacled underwater robot is proposed, characterized by a windmill-shaped array layout and a tangential wave-pushing type. The study explores the self-rolling propulsion achieved through this mode, systematically analyzing the hydrodynamic characteristics and quantitative relationships of this navigation state by varying the dimensionless oscillation frequency f* and the dimensionless maximum amplitude angle theta* of each tentacle. Utilizing overlapping grid technology, we developed a numerical solver capable of simulating autonomous motion with multiple degrees of freedom (DOFs) in OpenFOAM. The findings indicate that collaborative motion among multiple tentacles implements the concept of force cancellation, facilitating stable self-rolling propulsion. The instantaneous velocity curve exhibits a converging periodic trend. When f* is fixed, the cruising velocity coefficient CUx reaches its extreme value between theta* = 8 and 9, and the methods of increasing f* or theta* also continuously produce a gain effect on the roll performance. During the comparison with normal propulsion modes, we find that when the robot operates at low frequency and high amplitude, this invert mode can convert some hydrodynamic resistance into driving torque applied to the roll axis. This indicates that the reasonable energy utilization of self-rolling propulsion has greater advantages in improving cruising performance and enhancing maneuverability.

This paper conducts a numerical simulation to investigate the propulsion dynamics of an underwater vehicle outfitted with a dual cycloidal propeller configuration., Exploring the effects of blade spread length and rotational speed on the underwater performance of a navigational vehicle. First, the kinematic analysis of the vehicle is performed to determine the computational parameters of the hydrodynamic performance of the vehicle and to establish a simulation model of the vehicle. N ext, the simulation analysis of the vehicle under different conditions is carried out using CFD methods. The results show that the vehicle equipped with pairs of propellers can travel stably, and the rotational speed and blade spread length have a positive effect on the propulsion speed of the vehicle. The results provide an important reference for the theoretical research and physical development of the subsequent Cycloidal Propellers.

To address the limitations of bionic underwater vehicles (BUVs) in terms of surface operation capabilities, this study developed a robotic dolphin capable of achieving multiple underwater and surface motion modes, inspired by the vertical surface behavior of biological dolphins. By establishing a physical model of the dolphin and a multi-body cooperative kinematic model involving the body, caudal fin, and pectoral fins, numerical simulations were conducted to analyze the hydrodynamic performance of the dolphin's underwater horizontal motion and surface vertical motion. Based on these findings, the robotic dolphin's body module, caudal fin module, pectoral fin module, center-of-mass adjustment module, and corresponding control system were designed. Experimental studies on underwater/surface multi-mode motion were conducted using the constructed prototype platform, with results compared to numerical simulations. The results show that the robotic dolphin can achieve an underwater horizontal propulsion speed of 0.796 m/s and a horizontal turning speed of 19.51 degrees/s, as well as a surface vertical propulsion speed of 0.192 m/s and a vertical turning speed of 55.82 degrees/s. During underwater horizontal motion, the caudal fin provides the primary thrust, while the pectoral fins enable highly maneuverable in-place turning; in surface vertical motion, the caudal fin stabilizes the standing posture while the pectoral fins generate thrust for propulsion and torque for in-place turning. The center-of-mass adjustment module facilitates the transitions between postures. This study significantly enhances the surface operational capabilities of traditional BUVs, laying a scientific foundation for developing novel underwater/surface dual-mode operational robots.

To enhance the surface operation capabilities of bionic underwater vehicles and address challenges in surface information docking and energy replenishment, this study draws inspiration from a dolphin's surface inclined walking technique, focusing on inclined postures between horizontal and vertical orientations. The hydrodynamic performance of the bionic dolphin robot's inclined walking behavior at different inclined angles (e.g., 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 75 degrees) was investigated with single-caudal-fin and multi-fin propulsion modes. The quantitative models between kinematic parameters and hydrodynamic performance have been established. The transient evolution characteristics of the surrounding flow field were revealed, and key differences between the two modes were highlighted. The results demonstrate that kinematic parameters significantly influence the hydrodynamic performance of inclined walking behavior. Notably, the dolphin robot achieves optimal inclined walking efficiency (up to 69.8%) at inclined angles between 45 degrees and 60 degrees. The single-caudal-fin mode exhibits superior walking velocity and efficiency under high swing frequency and amplitude conditions but with greater variability. In contrast, the multi-fin mode demonstrates more stable hydrodynamic performance with smaller fluctuations. This study addresses the issue of traditional underwater robots being limited to a single underwater application scenario, providing a hydrodynamic basis for the design of underwater robots capable of surface operations.

The cycloidal propeller can adjust the thrust direction in real time by changing the blade's angle of attack, thus exhibiting excellent maneuverability. To explore and utilize the high maneuverability of cycloidal propellers, this paper analyzes the virtual camber effect generated during their rotation and proposes a method using flaps to compensate for the effect and enhance propeller performance. First, three models of cycloidal propellers-without flaps, with a single flap, and with double flaps-are constructed, and their propulsion processes are numerically simulated to verify the performance improvement brought by the blades with flaps. Second, a quantitative optimization study is conducted on the motion patterns of blade flaps to further amplify their enhancement effect. Finally, based on the quantitative study results, the optimal flap motion patterns for maximizing propulsion performance are summarized. The results confirm that the main thrust coefficient of blades with both leading-edge and trailing-edge flaps can be improved by up to 21.79 % compared to blades without flaps. After further optimizing the motion patterns of the leading-edge and trailing-edge flaps, the fluctuation in main thrust can be effectively reduced, and the main thrust coefficient can be increased to 23.64 %. The findings of this study provide a theoretical basis for the practical application of cycloidal propellers with flaps.

Based on cephalopods as the biomimetic prototype, a multi-tentacled underwater robot is developed. Each tentacle is controlled by a double-link mechanism, and its roll angle can be independently adjustable, highlighting its advantages in flexibility. With the collaboration between multiple tentacles, various driving modes including homologous mode, interlace mode, reverse mode and deviation mode can be formed, which can be matched with different application scenarios. Numerical simulation and prototype experiment are conducted on the self-swimming performance of robot, revealing the mechanism essence of composite modes from two aspects. The results indicate that the homologous mode follows the characteristics of fish-like waves, while the interlace mode can cause the robot to deviate from the initial route. The reverse mode, which has both explosive and efficiency-enhancing properties, is considered the most optimal choice for achieving propulsion. Regarding steering, the deviation mode generates stable yaw torque through continuous asymmetric swing of each tentacle. The underwater robot with high-frequency swing can significantly shorten the time to complete self-steering without affecting the turning radius (about 1.18 m). Multi-tentacled collaboration effectively solves the problem of single driving form in the field of underwater robots, and the overlapping motion mechanisms also contribute to meeting the diversity needs in underwater spaces.

This study conducts a numerical investigation of the self-propelled performance of a bio-inspired squid robot equipped with four rigid tentacles, exploring three sets of collaborative modes. Leveraging the open-source platform OpenFOAM, we develop a self-propulsion module incorporating the dynamic overset grid technique to manipulate the complex motion of rigid tentacles. The driving system of a single tentacle is simplified into a two-link mechanism, where the phase difference between the links effectively emulates the oscillatory pattern of fish-like locomotion. The interaction of four tentacles gives rise to three distinct driving modes: reverse, homologous, and interlace modes. The results indicate that the homologous mode follows the hydrodynamic characteristics of fish-like waves, the interlace mode can cause the robot to deviate from the initial path, and the reverse mode outperforms the other two modes, exhibiting a higher ultimate cruising speed. Regardless of the propulsion process, the cruising performance of the robot is significantly influenced by the maximum amplitude angle θmax. An increase in θmax also contributes to an elevation in the instantaneous longitudinal force coefficient CFx, with the most pronounced impact observed in the homologous mode. The disparity among the three modes is also evident in the periodic pressure variation and flow field evolution patterns. The vortex distribution during steady-state moments systematically reveals the collaborative effects among the tentacles in different modes on the self-propulsion performance.

In order to overcome the operational challenges faced by traditional underwater robots at the water surface and improve their surface maneuverability, this study adopts the dolphin as a biological model to investigate its hydrodynamic performance during the transition from standing-and-walking (SAW) to standing-and-turning (SAT) behaviors on the water surface. The research leverages the compound motion of the body, caudal fin, and pectoral fins in a three-dimensional dolphin physical model to realize three novel SAT modes based on the SAW, respectively: different amplitude (DA), different frequency (DF), and different phase (DP). Through a series of numerical simulations, the time-varying patterns of key parameters during the transition process were described, and the mapping relationships between kinematic parameters and hydrodynamic performance for each mode were quantitatively analyzed, revealing the transient evolution of the flow field around the dolphin model. The results showed that the proposed SAT modes could simultaneously generate vertical and horizontal thrusts, along with torque around the body's longitudinal axis, enabling the transition from SAW to SAT behavior. Among the three turning modes, the DF mode exhibited the most stable maneuverability. Notably, under specific conditions, the turning radius of the DP mode can reach 0.014 m, effectively achieving in-place SAT behavior, which is challenging for traditional underwater robots. This work provides a novel approach to addressing the surface operation challenges faced by traditional underwater robots, significantly enhancing their maneuverability on the water surface.

This paper proposes a novel amphibious robot with reusable cycloidal propulsion module, and conducts simulation and experimental studies on its propulsion performance. Underwater, the robot adopts the collaboration of multiple cycloidal propellers to achieve moving and turning, providing good maneuverability; on land, the four blades of each cycloidal propeller are used as the blade wheel to drive the robot to move and turn, thus realizing the reuse of the same propulsion module in two environments. The experimental results show that the robot's underwater movement speed coefficient reaches a maximum of 0.275; by changing the direction of the propellers on both sides, the robot can realize in-situ turning, and the turning speed increases with the increase of the revolution speed of the propeller. The robot multiplexes the cycloid propellers into blade wheels on land, moves with a maximum speed coefficient of 0.207, and realizes the functions of mobile turning and in-situ turning, respectively, by means of the speed difference between the blade wheels on both sides. The findings confirm the feasibility of the cycloid propeller as a reusable propulsion module for amphibious robots, and the developed robot has good maneuverability, which is promising for application in the field of amphibious operations.

Three different hovering modes, namely, the caudal fin, pectoral fins, and multi fins, were utilized to achieve the standing-and-hovering behavior in robotic dolphins. A three-dimensional dolphin model, consisting of body, caudal fin, and symmetric pectoral fins, was used as the virtual swimmer to implement three hovering modes. A novel paddling motion was proposed, and a symmetric shape was designed of the pectoral fins. The hovering mechanisms of different modes were revealed, and the mapping relationships between different motion and performance parameters such as hovering height, efficiency, stability, and rapidity were established. The respective advantages of the three hovering modes were compared. The results showed that the caudal fin mode had the best hovering stability, while the pectoral fins mode had the best hovering rapidity. Moreover, it is worth noting that the multi fins mode had both the good hovering stability and rapidity. Therefore, the optimal hovering mode and motion parameters can be selected based on different expected objectives to achieve the best results. This study provides a new approach to break through the spatial barriers to movement of underwater robots and provides a solid hydrodynamic theoretical basis for the development of cross-medium robots with multiple hovering modes.