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In contrast to other swimming modes, the motions of fins in the labriform mode can be categorized into the drag-based mode and the lift-based mode, which differ in terms of the thrust generation mechanisms. This variance in thrust generation mechanisms gives the labriform mode unique advantages in underwater propulsion. The term labriform indicates that propulsion occurs due to oscillatory movements of pectoral fins. Herein, to identify the key features of labriform locomotion, numerical simulations of a self-propelled biomimetic robotic fish with a Reynolds number (Re) of up to 3 000 000 in the labriform mode are performed. This study includes a detailed analysis of swimming performance and hydrodynamic mechanisms and their connection to three-dimensional vortex dynamics. Compared with the drag-based mode, the fish is observed to cruise faster and swim more smoothly in the lift-based mode. This study also finds that the pectoral fin can produce continuous thrust during one cycle in lift-based mode but can only generate thrust during the power stroke in the drag-based mode. By connecting vortex dynamics and surface pressure, the results show that the leading-edge vortices generated by pectoral fins are associated with most of the thrust production in both motion modes. The analysis of the vortex structure shows that the pectoral fins shed one vortex ring in one cycle of the drag-based mode and two vortex rings in one cycle of the lift-based mode. Our results provide new insights regarding the self-propelled swimming mechanism of biomimetic robotic fish with different labriform propulsion modes. [ABSTRACT FROM AUTHOR]

This research introduces a path planning methodology aimed at coordinating multiple unmanned surface vehicles (USVs) to accomplish target coverage task in obstacle-rich environments. Leveraging the locking sweeping method (LSM), our proposed method constructs task target point distance fields that integrates environmental constraints, alongside a task target point distance matrix. During the task allocation phase, we design a cost assessment function to assess the generated allocation solutions. And a greedy allocation strategy is employed to determine the optimal number of USVs required for mission completion, ensuring evenly distribution of target task points among them. Subsequently, we improve the ant colony optimization (ACO) method by redesigning the heuristic function and pheromone update rules, considering environmental constraints and task execution sequence constraints. This refinement facilitates the generation of optimized task execution sequences and safe navigation paths in obstacle environments. The effectiveness of the proposed method is validated through multiple sets of simulation experiments and compared with existing methods. The results demonstrate the practicality and efficacy of the method in addressing the challenges of USVs target coverage task in obstacle environments. • Based on the locking sweeping method, our constructs task target point distance fields, considering environmental constraints, and a corresponding distance matrix, enabling precise safe navigation planning in obstacle environment. • We employ a greedy allocation strategy to determine the optimal number of USVs for mission execution, ensuring equitable distribution of target points among USVs and maximizing mission efficiency. • By refining the heuristic function and pheromone update rules of the ACO method, the proposed method generates optimized task execution sequences and collision-free navigation paths, enhancing mission safety and effectiveness. • A cost assessment function has been devised, taking into account mission time, energy consumption, and resource costs, to assess the path planning outcomes of coverage solutions. [ABSTRACT FROM AUTHOR]

A wave-driven unmanned surface vehicle (WUSV) can sail on the sea simply by utilizing wave energy as its driving power. Recently, many studies have been performed on practical applications of oceanic scientific exploration by a WUSV, where the estimation criterion of the navigation performance of the WUSV is always its sailing velocity, although this method is very intuitive but not effective in analyzing the shortcomings in the design of the WUSV. On this basis, a new estimation method, which focuses on the efficiency of utilizing wave energy by the WUSV, is proposed in this paper. The theoretical contribution of this paper is that a generalized solution function for the efficiency of utilizing wave energy is obtained and is expressed in the form of the product of several non-dimensional parameters. In detail, algorithms to calculate input power, i.e., wave energy harvested by the surface float, and usable power, i.e., the product of sailing velocity and thrust force, have been developed. The other development presented in this paper is the proposal of some effective ways to improve the ability of the WUSV to utilize wave energy. Then, numerical simulations are carried out to validate the proposed method, and corresponding experiments are employed to investigate the performance of the method. The results show that the four major factors to impact the ability of the WUSV utilize wave energy are the thrust force of the propulsor, the sailing velocity of the WUSV, the principal dimensions of structures and the wave environment parameters. Summarizing the analysis in numerical validations and experimental studies, the WUSV can generally achieve higher efficiency in utilizing wave energy when it sails in a specific wave environment with certain parameters. Meanwhile, the utilization efficiency of the WUSV is not always much higher, and the reasons for this phenomenon mainly include two aspects: (1) most of the wave energy harvested by the surface float is used to work against the calm water resistance of the propulsor and the umbilical cable; and (2) based on the current design, the motions of the surface float cannot fully respond to the waves. Therefore, an integrated optimizing design for the propulsive and resistance performance of the propulsor, as well as the motion response and resistance performance of the float, will be an effective way to improve the ability of the WUSV to utilize wave energy. In addition, the differences in the sailing velocity and heaving amplitude of foils cause different curve trends for the utilizing efficiency between numerical simulations and experiments. [ABSTRACT FROM AUTHOR]

This article studies the two‐layer leader‐follower optimal affine formation maneuver (AFM) control problem for multiple unmanned surface vessels (USV). The basic idea of the developed two‐layer control framework is to realize the time‐varying formation for the leader group in advance, and then let the follower USVs agilely respond to the target formation affinely localized by leaders. To realize this goal, a time‐varying formation reference system is first synthesized to describe leaders' target formation in the affine map of the nominal configuration. The reference system is set up by virtual vessel velocity command and unrotated geometric command, which make it possesses an intuitive application structure for operators to plan AFM commands in real‐time sensed surrounding environments. Then, by means of optimized backstepping design and the adaptive dynamic programming technique, actor‐only reinforcement learning controllers are established for USVs in both two layers to realize the optimal target formation tracking objective. For the proposed optimal controllers, the persistent excitation condition that is required by common optimal control schemes is also removed, which endows the presented scheme with lower computational complexity. Finally, theoretical analysis and simulation results illustrate the closed‐loop stability and the effectiveness of the presented scheme. [ABSTRACT FROM AUTHOR]

Unmanned surface vehicles (USVs) have wide applicability owing to their high velocities, intelligence, and unmanned operational characteristics, and therefore, they have garnered considerable research interest. To accommodate complex maritime environments comprising static/dynamic obstacles, a novel efficient path-planning algorithm called the constrained locking sweeping method and velocity obstacle (CLSM-VO) is developed. This algorithm uses the locking sweeping method (LSM) to provide the initial yaw angle for the velocity obstacle (VO) algorithm. While retaining the performance of the VO algorithm, the initial yaw angle based on the LSM can help the VO algorithm optimise the global search performance. Compared with conventional path-planning algorithms, the proposed algorithm demonstrates higher computational efficiency and is applicable to complex dynamic environments with multiple moving obstacles. Additionally, a constraint layer is designed, by combining the proposed algorithm with the yaw-velocity performance of the Tianxing-1 USV, such that the generated path exhibits better smoothness. The adjustable avoidance distance calculated by the proposed algorithm can help prevent collision accidents compared to that by existing algorithms. To evaluate the algorithm performance, particularly the capacity to handle complex dynamic environments, multiple simulations are performed in environments built using electronic nautical charts. The results indicate that the proposed algorithm can effectively handle complex maritime traffic scenarios by generating smooth and safe trajectories. • A constrained locking sweeping method and velocity obstacle based path planning algorithm for USVs is proposed, which has a higher computational efficiency and is applicable to complex dynamic environments with multiple moving obstacles. • A new environment representation method is proposed to improve the computational performance of the planner and reduce the amount of information disseminated in the communication system. • A new collision model of the USVs is designed to cope effectively with complex maritime traffic scenarios. • A constraint layer is designed by obtaining the real USV yaw-velocity performance data to further generate smooth and safe trajectories. [ABSTRACT FROM AUTHOR]

This paper presents the simulated and experimental results of propeller-rudder systems with propeller boss cap fins (PBCFs) and analyzes the hydrodynamic performance of PBCFs in propellerrudder systems. The purpose is to study the impact of PBCFs on the hydrodynamic performance of rudders. Hydrodynamic experiments were carried out on propeller-rudder systems with PBCFs in a cavitation tunnel. The experimental energy-saving effect of the PBCF without a rudder was 1.47% at the design advance coefficient J = 0.8. The numerical simulation was based on the Navier-Stokes equations solved with a sliding mesh and the SST (Shear Stress Transfer) k-ω turbulence model. After the grid independence analysis, the flow fields of an open-water propeller with and without a PBCF were compared, then the efficiencies of the propulsion systems including different rudders and the thrust coefficient Kr of rudders were analyzed. The results indicate that the installation of a PBCF increases the resistance of the rudder, which results in a reduction in the energy-saving effect of the PBCF. At the design advance coefficient, the energy-saving effect of the PBCF with an ordinary rudder and a twisted rudder decreases from 1.47% to 1.08% and 1.16%, respectively; thus, it is important to factor in the rudder of a propulsion system when evaluating the energy-saving effects of PBCFs . [ABSTRACT FROM AUTHOR]

For the autonomous underwater vehicle (AUV) platform, state quantization is a widely existing phenomenon as the result of I/O signal conversion between hardware module and control module. This article considers such an adaptive trajectory tracking control problem for AUVs with the effects of quantized state feedback and ocean disturbances. To ensure the stability of the closed-looped system under quantized states and unmeasurable ocean disturbances, a novel quantized extended state observer (QESO) is firstly proposed. Then, an adaptive anti-disturbance control scheme is developed to achieve the trajectory tracking control goal. The main advantage of the proposed algorithm paper lies in that the resulted controller is established without any usage of continuous states. Besides, rigorous stability analysis validates the uniformly ultimately boundedness of the resulting closed-loop system under quantized states. Simulation experiments demonstrate the effectiveness and robustness of the proposed algorithms. • The trajectory tracking control problem of AUVs is solved under the effects of states quantization. • A novel quantized extended state observer is proposed to approximate the unknown disturbances. • A backstepping controller is proposed to solved the quantized state feedback control problem. [ABSTRACT FROM AUTHOR]

The form-changeable boats have received much attention due to its far-ranging applicability, strong combat capability and the low operating cost. This paper constructs a variable-structure boat whose mainhull is planing hull form and twin retractable deep-V side-hulls are placed on both sides of the mainhull, aims at assessing its seakeeping performance in the monomer-form state (MFS) and the trimaran-form state (TFS) by model tests, numerical simulation and sea trials. Regular head wave tests were conducted at volume Froude numbers (F r ∇) ranging from 0.44 to 2.67 for the MFS and TFS models. Verification and validation of the CFD method were performed, the influences of side-hull draft on the seakeeping of TFS model under different wave configurations were analysed. Sea trials of the full-scale vessel (USV) in irregular sea were also carried out at the design velocities. The results show that the TFS has different effects on motion responses and vertical accelerations of the model at different speeds and wavelengths, the applicable range of side-hulls with gain effect is in the diaplacement and semi-planing stage, and the maximum velocity is suggested not to exceed F r ∇ = 2.22. For the USV at F r ∇ = 1.33, the TFS mainly cut down its roll in irregular head sea, followed by following and beam sea; when the F r ∇ increases to 1.78, the roll gain declines, but the pitch gains more, the better side-hull draft ratios are 1.0 and 0.67, corresponding to the above two design speeds, respectively. • A variable-structure boat with retractable twin deep-V side-hulls are constructed. • Seakeeping performance of the boat in the monomer-form state (MFS) and trimaran-form state (TFS) is assessed. • The influences of side-hull draft ratio on motion responses of the TFS are analysed. • The sea trials provide the better side-hull draft ratios of the full-scale USV in the TFS. [ABSTRACT FROM AUTHOR]

This article tackles the finite‐time global trajectory tracking control problem of the autonomous underwater vehicle (AUV) in presence of input saturation constraints, actuator faults, unknown dynamics, and external disturbances. First, we describe the orientation of the AUV by rotation matrix instead of classical Euler angle or unit quaternion such that the AUV's dynamics could be globally formulated without singularity and unwinding phenomenon. After that, a smooth dead zone‐based model is introduced here to linearize the actuator model, leaving that the adaptive laws could be suitable for the solution of input saturation and actuator faults. Considering that the difficulty of model dynamic acquirement, together with the complicity of rotation‐matrix‐based representation, would trouble deployment of the controller. The minimum learning parameter technology is thereby utilized to approximated the dynamic nonlinearity of the AUV. On the basis of these, a rotation‐matrix‐based sliding mode control scheme is technically proposed. It is proved that the tracking errors can be stabilized to a small neighborhood of origin within a settling time. Finally, several set numerical experiments are conducted to assess the effectiveness and show the advantages of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In maritime engineering, ensuring vessel stability remains a paramount concern. This study investigates the hydrodynamic response of Magnus anti-rolling devices, modeled as swinging or slewing rotating cylinders, under a ship's rolling motion. Through numerical simulations using the overset mesh technique and large eddy simulation, we analyze various parameters, including rolling angles, rotating speeds, and swinging amplitudes. Our findings highlight the importance of considering the ship's degree of freedom as substantial ship rolling significantly affects hydrodynamic coefficients on the rotating cylinder. We observe interesting dynamics during slewing motion, with the cylinder forming a spiral tip vortex. Optimizing the cylinder's rotating speed enhances the lift-to-drag ratio, particularly for small rolling angles. Furthermore, the effective lift generated during swinging motion is lower than during slewing motion, emphasizing the need to optimize the swinging amplitude, which is recommended to be no less than 170°. These insights advance our understanding of Magnus anti-rolling devices and offer practical guidance for improving vessel stability in complex maritime environments. [ABSTRACT FROM AUTHOR]

The finite-length rotating column has been extensively studied because of its importance in various fields, such as marine and aerospace. In this study, the hydrodynamic performance of a finite-length rotating column with two free ends at different angles of attack is investigated using a large eddy simulation method. The effects of various geometries (including an equal-section cylinder and a variable-section truncated cone), incoming flow velocities, column rotation speeds, and angles of attack on the lift and drag characteristics and wake field of the rotating column are analyzed. The results reveal that a free end creates a concentrated tip vortex, which shortens the effective length that can generate the Magnus effect. Across different geometries and computational conditions, a relatively consistent lift coefficient is found for angles of attack from 60° to 120°, with the cone design significantly reducing the drag by approximately 10% for angles of attack from 120° to 150°. These findings provide valuable insights into the practical application of finite-length rotating columns. Specific recommendations for optimizing the design of these columns are suggested, including choosing appropriate geometries and considering the effects of incoming flow velocities and column rotation speeds. [ABSTRACT FROM AUTHOR]

Tuna, dolphin and whale as typical thunniform mode fish have obvious differences in body size and caudal fin shape. As the main propeller, the difference in the shape of caudal fin will have a significant impact on the swimming performance of fish. In this work, numerical simulations are adopted to study the hydrodynamics of self-propelled thunniform swimming. By changing the trailing-edge profile parametrically, three different tails varying from forked tuna-like tail to the square-shaped tail are constructed. The results revealed that the tuna-like tail is most beneficial to fish long distance cruise, while the square-shaped tail is the most unfavorable. It is interesting to note that for tuna-like tail, decreasing the undulating amplitude can improve fish's long-distance cruising ability more than decreasing the pitching amplitude, but the opposite is true for the square-shaped tail. Further study on the vortex dynamics and the surface pressure distribution indicate that the backward expansion of the trailing-edge profile will delay the shedding of the leading-edge vortex and increase the existence time of low-pressure region on the caudal fin. Since the pitching angle is reduced synchronously, the delayed disappearance of the low-pressure region will generate more lateral force. This will lead to more energy loss. • Numerical simulations of a fish-like swimmer with three different caudal fins are carried out based on computational fluid mechanics method. • The swimming performance, hydrodynamics force, pressure field and wake structure of the swimmer are analyzed in detail. • The pressure distribution on the swimmer surface is visualized to investigated the mechanisms of fluctuations of forces. • 3-D wake structures are visualized using the iso-surfaces of q -criteria and 3-D streamlines to investigate the vortex dynamics. [ABSTRACT FROM AUTHOR]

Abstract A large eddy simulation based on the lattice Boltzmann method is used in this study to develop a computation program for fluid flow past an elliptic cylinder near a moving wall. The flow field around the elliptic cylinder as it approaches the moving wall is simulated for different axis ratios at Reynolds numbers of 200 and 400. The effects of the gap ratio, axis ratio, and Reynolds number on the flow field, force coefficient, and Strouhal number are investigated. The moving wall inhibits the normal velocity and stabilizes the flow, thereby suppressing vortex shedding. This study focuses on the mechanisms that various parameters influence the vortex shedding of flow past an elliptic cylinder near a moving wall. The calculation results show that decreasing the gap and axis ratios both suppress vortex shedding, while the increase in Reynolds number effectively alleviates the complete suppression of vortex shedding. A decreased axis ratio increases the gap ratio for vortex shedding suppression, and an increased Reynolds number significantly lowers the gap ratio for complete vortex shedding suppression. As the gap ratio decreases, owing to the interaction between the positive vortex separated from the lower side of the cylinder and the negative vortex separated from the moving wall, the strength of the positive vortex formed at the lower side of the cylinder gradually decreases downstream. This interaction is more prominent at small axis ratios. When the Reynolds number increases, the interaction between the positive vortex separated from the lower side of the cylinder and the negative vortex separated from the moving wall is noticeably weaker, and the positive vortex maintains its strength for a longer distance downstream. The computed results are in good agreement with the limited experimental data published in literature. Highlights • A MRT Lattice Boltzmann method based on large eddy simulation was applied. • The mechanisms of the vortex shedding from an elliptic cylinder near a moving wall was investigated. • A decrease in axis ratio increases the gap ratio for vortex shedding suppression. • An increase in Reynolds number effectively alleviates the complete suppression of vortex shedding. [ABSTRACT FROM AUTHOR]

This article studies the secure formation control problem for autonomous surface vessels (ASVs) with directed event‐triggered communication subject to denial‐of‐service (DoS) attacks. DoS attack is a type of cyber‐attacks meant to cut down a network, thus causing legitimate neighbors to lose their expected communication in the control mechanism. A distributed secure formation control algorithm (DSFCA) is proposed to mitigate the negative impact of DoS attacks. Specifically, a model‐based dynamic event‐triggered mechanism (DETM) with positive minimum inter‐event times (MIET) is developed to save communication resources. Wherein, a clock‐like dynamic is chosen to guarantee the non‐Zeno behavior and provide a positive MIET guarantee. Simultaneously, a model‐based state estimator (MBSE) is designed in DETM to reduce the effect of the error caused by DETM during the triggering interval. Furthermore, the effects of DoS attacks on the formation control systems are analyzed. Some sufficient conditions are derived, in which the upper bounds of attack duration rate and attack frequency are given. Finally, numerical simulations are provided to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In practical maritime conditions, ship hulls experience heave motion due to the action of waves, which can further drive the ship’s propellers to oscillate relative to the surrounding water. In order to investigate the motion of a propeller working behind a surface vessel sailing in waves, a numerical simulation is conducted on a propeller impacted by heave motion in cavitating flow using the Reynolds-averaged Navier-Stokes (RANS) method. The coupling of the propeller’s rotation and translation is fulfilled using equations of motion defined for this purpose. The heave motion is simplified as a periodic motion based on a sinusoidal function. The numerical transmission of information from the unsteady flow field is achieved using the overset grid approach. In this manner, the unsteady thrust coefficient and torque coefficient of propellers in different periods of heave motion are analyzed. A comparative study is implemented on the unsteady cavitation performance and wake characteristics of propeller. With the propeller’s heave motion, the flow field non-uniformity constantly changes the load on the propeller during each revolution period and each heaving period, the propeller load and the wake field are closely related to the variation of heave motion period. The results obtained from the numerical simulation are expected to serve as a useful theoretical reference for the numerical analysis of a propeller in a heave motion. [ABSTRACT FROM AUTHOR]

Wang, L.; Guo, C.; Su, Y.; Wu, T., and Wang, S., 2017. Numerical study of the viscous flow field mechanism for a non-geosim model. Inasmuch as the scale effect of wake fields has not yet been resolved, the application of a 'Smart Dummy' model in predicting the full-scale nominal wake field and propeller-induced hull pressure fluctuations of ships sailing in coastal environments and zones farther out to sea has shown great potential since the concept was first proposed. However, it still has not been widely used because the rules and mechanisms of the Smart Dummy model are seldom studied by international and domestic researchers. A non-geometrically similar model (non-geosim model) of the KRISO3600TEU Container Ship (KCS) was designed by employing a local parameterization modeling method based on computational fluid dynamics numerical simulation methods. Based on the viscous flow field mechanism, numerical analysis was performed on the ship resistance performance, free-surface wave pattern, hull volume-of-fluid distribution, hull streamlines, nominal wake field at the propeller plane, and stern axial velocity field. First, the accuracy of meshing and numerical methods was verified using grid-independent analysis. Subsequently, the same grid and methods were applied for the numerical simulation and analysis of towed bare hull and self-propelled ships. The results show that the free-surface wave pattern of the Smart Dummy model was almost identical with that of the geosim model, and the distribution of boundary layers around the hull shows that the Smart Dummy had thinner and more contracted velocity boundary layers after the deformation area. The stern streamline distribution further indicated that the Smart Dummy model had a more uniform propeller inflow velocity. The difference in the angle of attack of the propeller blade operating at the rear of the KCS model accounted for the overprediction of the propeller-induced hull pressure fluctuations using the geosim model. Overall, the new method proposed in this paper has important potential applications in predicting flow characteristics for large container ships sailing in coastal environments and zones farther out to sea. [ABSTRACT FROM AUTHOR]

Numerical simulations are employed to study the hydrodynamics of self-propelled thunniform swimming. The swimmer is modeled as a tuna-like flexible body undulating with kinematics of thunniform type. The wake evolution follows the vortex structures arranged nearly vertical to the forward direction, vortex dipole formation resulting in the propulsion motion, and finally a reverse Kármán vortex street. We also carry out a systematic parametric study of various aspects of the fluid dynamics behind the freely swimming behavior, including the swimming speed, hydrodynamic forces, power requirement and wake vortices. The present results show that the fin thrust as well as swimming velocity is an increasing function of both tail undulating amplitude Ap and oscillating amplitude of the caudal fin θm. Whereas change on the propulsive performance with Ap is associated with the strength of wake vortices and the area of suction region on the fin, the swimming performance improves with θm due to the favorable tilting of the fin that make the pressure difference force more oriented toward the thrust direction. Moreover, the energy loss in the transverse direction and the power requirement increase with Ap but decrease with θm, and this indicates that for achieving a desired swimming speed increasing θm seems more efficiently than increasing Ap. Furthermore, we have compared the current simulations with the published experimental studies on undulatory swimming. Comparisons show that our work tackles the flow regime of natural thunniform swimmers and follows the principal scaling law of undulatory locomotion reported. Finally, this study enables a detailed quantitative analysis, which is difficult to obtain by experiments, of the force production of the thunniform mode as well as its connection to the self-propelled swimming kinematics and vortex wake structure. The current findings help provide insights into the swimming performance and mechanisms of self-propelled thunniform locomotion. [ABSTRACT FROM AUTHOR]

Zhao, D.; Guo, C.; Su, Y.; Wang, L., and Wang, C., 2017. Numerical study on the unsteady hydrodynamic performance of a four-propeller propulsion system undergoing oscillatory motions. In this study, the hydrodynamic performance of single- and four-propeller propulsion systems were numerically calculated under open-water and oscillating conditions to elucidate the effect of oscillatory motions on the hydrodynamic performance of propellers of large vessels under wave conditions. The Reynolds-averaged Navier-Stokes method and self-defined motion equations were adopted to describe the coupled rotation/oscillation of propellers in order to simulate their motion with the ship body under actual sea conditions. A numerical simulation of the unsteady flow fields was performed by using an overset-grid approach. The unsteady thrust coefficient, torque coefficient, propeller wake vortex, and blade pressure distribution during the coupled self-rotation and heaving oscillation of the propellers were analyzed at different speeds of advance. The propulsive efficiency and received power of the propellers were calculated and analyzed for various conditions, with a focus on the effect of different oscillation frequencies on the hydrodynamic performance of multipropeller propulsion systems. The results presented in this paper are theoretically meaningful and may be applicable to optimizing the propeller design or arrangement of multiple propellers, predicting the hydrodynamic performance of multipropeller propulsion systems, and rationally determining the power allocation of main engines. This paper provides theoretical support for resolving practical engineering problems. [ABSTRACT FROM AUTHOR]

Ship anti-rolling devices are an essential component of modern vessels. The core component of the Magnus effect-based ship anti-rolling device is a rotating cylinder, hereinafter referred to as the Magnus cylinders. In this paper, fully parametric three-dimensional modeling of Magnus cylinders was performed, and the design space dimension was reduced using the Sobol design optimization method while still providing accurate and reliable results. The Sobol method generates quasi-random sequences that are more uniformly spaced in the search space and can more efficiently cover the entire solution space. The shape optimization study of the Magnus cylinder was carried out in conjunction with the computational fluid dynamics method to find the geometry of the Magnus cylinder with excellent hydrodynamic performance. Critical design parameters include the diameters of the cylinder ends and the length of the cylinder. The hydrodynamic and flow field characteristics of Magnus cylinders before and after the optimization were compared. The results show that there can be multiple local optimal values for lift and drag of Magnus cylinders within the design space to increase the lift and decrease the drag. The Magnus effect primarily influences the position of the vortex-shedding separation point at the surface of Magnus cylinders and deflects the wake to one side. For the optimized Magnus cylinder, the distribution of pressure and velocity in the flow field is significantly different. This research forms the basis for improving the practical application of Magnus anti-rolling devices. [ABSTRACT FROM AUTHOR]

Zhao, D.; Guo, C., and Su, Y., 2016. Hydrodynamic performance of ichthyoid rudder at different rudder angle settings. This paper presents the first calculation of the open-water hydrodynamic performance of ordinary and ichthyoid rudders in a viscous flow field using the computational fluid dynamics method as well as an investigation of the lift and resistance coefficients of the two rudders. First, an ichthyoid rudder was formed by adjusting the shape of an ordinary rudder to mimic the shape of a fish tail. Subsequently, open-water tests of ordinary and ichthyoid rudders were conducted, and the maneuverability of the two rudders was compared at different rudder angle settings. The results of the theoretical calculations show that the ichthyoid rudder has obvious advantages over the ordinary rudder in improving the lift coefficient and postponing the stall phenomenon at greater critical rudder angles. This paper also presents calculations of the force and moment of the two rudders at different rudder angle settings and compares the results of these calculations. By comparing the wake fields of the two rudders at different rudder angle settings, our study provides a theoretical basis for the improved ship maneuverability observed with the ichthyoid rudder. [ABSTRACT FROM AUTHOR]

In this study, we present an artificial intelligence control method specifically designed for the Magnus anti-rolling device. The core of our approach is the development of a co-simulation framework that integrates an intelligent algorithm with a three-dimensional numerical computational programme within a complex hydrodynamic environment. This integration is achieved through the use of a deep reinforcement learning algorithm, which allows for intelligent adaptation of the anti-rolling device's rotating speed in real time. Our research focuses on evaluating the performance of the intelligent anti-rolling control algorithm under a variety of conditions, including different ship-model roll angles, column geometry models, and varying swinging or slewing speeds. Through numerical studies, we compare the effects of these variables on the effectiveness of the control method. The co-simulation technology provides a platform for testing the intelligent control method. It allows a detailed examination of how the intelligent algorithm interacts with the hydrodynamic responses of the Magnus anti-rolling device, ensuring that the control method can adapt to a wide range of operational scenarios. This adaptability is crucial for maintaining stability and improving the performance of the anti-rolling device in real maritime environments. This study highlights the potential for integrating artificial intelligence with traditional maritime engineering solutions. • Co-simulation framework integrates hydrodynamic responses with intelligent control. • An Intelligent control method for the Magnus anti-rolling device was developed. • Real-time adaptation of rotating speed via deep reinforcement learning. • Performance was evaluated across various roll angles and geometries. • Enhances stability and adaptability in diverse maritime environments. [ABSTRACT FROM AUTHOR]

The form-changeable unmanned surface vehicle (USV) have a high research enthusiasm due to its far-ranging applicability, strong combat capability and low operating cost. This paper constructs a variable-structure Small Waterplane Area Twin Hull (SWATH) USV with twin hydrofoils, and aims at assessing its longitudinal motion stability (LMS) in the catamaran-form state (CFS) by parametric model and CFD method. The parametric representation of the torpedo, double struts and hydrofoils was conducted by uniform B-spline curves, 3-D coordinates conversion and surface generation method. The CFD code was verified, and used to calculate the LMS of the vessel with different strut configurations, form-states and CG position. Based on numerical results, the causes of different instability behaviors were expounded. Further, the LMS of the models with and without hydrofoils was compared at F r = 0.1–0.8, the adjustment method of fore and aft hydrofoils to sailing attitudes was explained, the rational allocation of attack angles was also offered. The results show that the slender double struts arranged at both ends of torpedo is more conducive to the LMS and resistance, the vessel should sail in the CFS of γ s ≤ 10° at F r > 0.6; the hydrofoils slightly weakens the LMS at low F r , makes the bow appear slight updip at F r = 0.4–0.6, but still can sails stably; adjusting attack angles can inhibit the instability, and the most effective allocation of the fore and aft hydrofoil's attack angles is linearly arranged at F r = 0.7, 0.8, also the adjusting can reduce the upward trim at F r = 0.9, 1.0, but the hull is lifted higher and arises the bow updip due to the free surface suction force, at this time, the attack angle's graduation should be refined to realize the finer adjustment to the dynamic trim. • A form-changeable SWATH USV with twin hydrofoils is designed, and the longitudinal motion stability of its catamaran-form state (CFS) is investigated by the CFD method. • Parametric modeling of the form-changeable CFS model with different double struts is established by uniform B-spline curves, 3-D coordinates conversion, surface generation method. • The different instability behaviors of the CFS models with different struts, form-states are analysed by the captured flow details and hydrodynamic pressure nephogram. • The adjustment method of fore and aft hydrofoils to sailing attitudes is studied, also the rational allocation on attack angles to inhibit the instability is offered. [ABSTRACT FROM AUTHOR]

Underwater terrain matching navigation technology is an important research area for the underwater navigation of Autonomous Underwater Vehicles (AUVs). Terrain matching navigation can realise long-term, subtle, all-weather, and high-precision underwater AUV navigation. In this paper, the research status of the application of AUV underwater terrain matching navigation is reviewed, the system composition, theory and terrain matching methods of underwater terrain matching navigation are summarised and the advantages of a multi-beam bathymetric system in underwater terrain matching navigation are discussed. The current research thoughts are summarised, the key issues are pointed out, and possible future development trends are discussed. [ABSTRACT FROM AUTHOR]

This article investigates the distributed event‐triggered affine formation maneuver control problem for multiple underactuated surface vessel (USV) systems with positive minimum inter‐event times (MIET). Unlike common formation control methods that only ensure the formation system to keep a fixed geometric shape, the proposed scheme enables multiple USVs maneuver as a group in translation, shearing, rotation, or combinations of them. The proposed control strategy is composed of two parts. First, a distributed event‐triggered observer (DETO) is proposed to observe the time‐varying target formation under the lack of global leaders' information and govern the inter‐vessel communications among the formation group. Besides, the MIET of the designed communication strategy is proved to be strictly positive and computable from formation configurations and control parameters. With these guarantees, the maximum communication frequency can be known in advance. Then, upon the DETO signals, the adaptive local tracking controller is subsequently synthesized for each vessel to realize the target formation track. Rigorous theoretical analysis and simulation results are finally conducted to verify the validness and effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Under zero-speed conditions, ships are particularly susceptible to the effects of waves, which directly impact the safety of the vessel. A ship anti-rolling device based on the Magnus effect is designed to mitigate rolling motions across a full range of speeds, thereby enhancing the vessel's stability. This study presents an experimental investigation and intelligent control of Magnus anti-rolling devices aimed at enhancing ship stability at zero speed. The test setup, intelligent control algorithm, and experimental procedures specifically tailored for evaluating the Magnus anti-rolling device were designed. Following this, a comprehensive analysis was conducted to assess the effects of different cylinder geometries, swinging speeds, initial roll angles, and control methods on the anti-rolling characteristics of the device. Results demonstrate that the intelligent control method achieves an average anti-rolling efficiency of 89%. Additionally, the optimised geometric model of the Magnus anti-rolling device exhibits improved anti-rolling efficiency relative to the original model. The study confirms the stability and robustness of the intelligent Magnus anti-rolling device and suggests future research directions for practical applications aboard full-scale vessels in complex marine environments. • Magnus effect-based anti-roll device enhances ship stability. • The experimental study validates intelligent control's effectiveness. • Various factors: cylinder geometry, swinging speed, and initial roll angle. • The optimised geometric model improves device performance. • The results provide a reference for studies on practical application at sea. [ABSTRACT FROM AUTHOR]

Anti-rolling devices are widely used in modern shipboard components. In particular, ship anti-rolling control systems are developed to achieve a wide range of ship speeds and efficient anti-rolling capabilities. However, factors that are challenging to solve accurately, such as strong nonlinearities, a complex working environment, and hydrodynamic system parameters, limit the investigation of the rolling motion of ships at sea. Moreover, current anti-rolling control systems still face several challenges, such as poor nonlinear adaptability and manual parameter adjustment. In this regard, this study developed a dynamic model for a ship anti-rolling system. In addition, based on deep reinforcement learning (DRL), an efficient anti-rolling controller was developed using a deep deterministic policy gradient (DDPG) algorithm. Finally, the developed system was applied to a ship anti-rolling device based on the Magnus effect. The advantages of reinforcement learning adaptive control enable controlling an anti-rolling system under various wave angles, ship speeds, and wavelengths. The results revealed that the anti-rolling efficiency of the intelligent ship anti-rolling control method using the DDPG algorithm surpassed 95% and had fast convergence. This study lays the foundation for developing a DRL anti-rolling controller for full-scale ships. [ABSTRACT FROM AUTHOR]

This paper is devoted to the problem of prescribed performance trajectory tracking control for symmetrical underactuated unmanned surface vessels (USVs) in the presence of model uncertainties and input quantization. By combining backstepping filter mechanisms and adaptive algorithms, two robust control architectures are investigated for surge motion and yaw motion. To guarantee the prespecified performance requirements for position tracking control, the constrained error dynamics are transformed to unconstrained ones by virtue of a tangent-type nonlinear mapping function. On the other hand, the inaccurate model can be identified through radial basis neural networks (RBFNNs), where the minimum learning parameter (MLP) algorithm is employed with a low computational complexity. Furthermore, quantization errors can be effectively reduced even when the parameters of the quantizer remain unavailable to designers. Finally, the effectiveness of the proposed controllers is verified via theoretical analyses and numerical simulations. [ABSTRACT FROM AUTHOR]

This paper is dedicated to solving the time-varying formation control problem of multiple marine surface vehicles experiencing uncertainties, actuator faults and saturation constraints. At first, the transformed Euler-Lagrange dynamics of MSVs are presented for problem formulation. Based on such dynamics, a novel integral manifold possessing finite-time reachability is designed, and it follows that a time-varying formation tracking control scheme is proposed. Owing to the favorable properties of the Euler-Lagrange systems and the hyperbolic tangent function, amplitude constraints on the command control signals could be satisfied by just tuning the design parameters instead of complicated anti-saturation design. Then, the adverse effect arisen from uncertainties and actuator faults is suppressed by designed adaptive laws. Through Lyapunov stability analysis and homogenous theory, the globally finite-time stability of the closed-loop system is theoretically confirmed. Finally, numerical simulations are carried out to show the efficacy of the proposed algorithm. • A novel integral sliding mode surface is proposed to endow the formation system with finite time stability. • Time-varying formation problem will be handled in the presence of input saturation constraints and actuator faults. • The formation system will possess satisfied robustness under the presented method. [ABSTRACT FROM AUTHOR]

This paper provides a robust fixed-time trajectory tracking controller for underactuated surface vessels (USVs) suffering from unmodeled dynamics and external disturbances. Initially, a novel model transformation is firstly applied for the possible application of sliding mode control technology. Then, fixed-time convergence for tracking errors can be guaranteed by stabilizing the transformed system. During the design process, a constructive sliding mode surface is structured with the application of hyperbolic tangent function, which could ensure the settling time of the designed system independent of initial states. To accommodate unknown system dynamics and perturbations, the Minimum-Learning-Parameter based neural network and adaptive updating laws are adopted. Theoretical analysis shows that tracking errors enjoy practical fixed-time stability under the proposed controller. Numerical simulation results illustrate the effectiveness and superiority of the proposed control scheme. •An integrated fixed-time tracking controller is developed for USVs based on sliding mode technology. •A novel sliding mode surface is devised to ensure fixed-time stability. •System uncertainties and disturbances will be handled via neural network and adaptive laws. [ABSTRACT FROM AUTHOR]

Rudder added thrust fins have gained popularity as an energy saving device (ESD) by virtue of; simple design, easy installation and its effectiveness. The current work seeks to investigate the influence of geometric parameters and mounting orientation on the energy saving effect of rudder attached thrust fins based on numerical analysis. Open water, bare hull resistance and self-propulsion simulations were carried out by solving the Reynolds-Averaged-Navier-Stokes Equations (RANSE) using a Computational fluid Dynamics (CFD) code. In the case of geometrical parameters, hydrofoil camber and fin span were varied. The fin orientation was then studied by varying the mounting direction in relation to the angle of attack and the location of pressure and suction sides. A validation study carried out showed good agreement between CFD and model tests. It was noted that the rudder thrust fin is sensitive to the hydrofoil orientation. Also, with the right hydrofoil orientation, the fin performance is optimum at an angle of attack of about 2°. This study presents a design guideline for rudder thrust fins to ensure optimum energy gains. • Data validation shows good agreement between the CFD results and experimental data. • Rudder thrust fins are sensitive to hydrofoil camber. • Hydrofoil orientation influences performance of thrust-fins. • Thrust fins can be bettered by having some angle of attack. [ABSTRACT FROM AUTHOR]

The form-changeable unmanned surface vehicles (USVs) have garnered considerable research interest due to their wide-ranging applicability and robust combat capabilities. This paper specifically addresses the longitudinal motion stability of a variable-structure Small Waterplane Area Twin-hull Ship (SWATH) equipped with twin hydrofoils, aiming to evaluate the impact of hydrofoils on sailing attitudes. Initially, model tests were systematically conducted under different form-states, both with and without hydrofoils, in calm water conditions. Subsequently, verification and validation of computational fluid dynamics (CFD) simulations were performed against the experimental results. Finally, strategies to mitigate bow-diving during high-speed tests were proposed based on the validated CFD simulations, focusing on adjusting hydrofoil attack angles. The test results indicate that with outwardly spreading struts, the bow tends to dip down more as speed increases. Model tests, combined with CFD simulations, serve as a reference for stable sailing at full speeds, presenting strategies for adjusting attack angles under varying form-states and speeds. Moreover, effective foil adjustment modes are identified to suppress bow-diving, considering thrust. Strategies for adjusting attack angles during the acceleration process are summarized, verifying the stability of the optimal 10° form-state at high speeds. • The towing tests of a variable-structure SWATH model in calm water were systematically carried out. • Verification and validation of the CFD were achieved, model tests and the CFD jointly provided a form-state reference on the stable sailing of the vessel. • The adjusting strategies of hydrofoil's attack angles under the changing form-states and the variety of speeds were presented. • The adjusting modes of foils were effective to inhibit the bow-diving, the adjustment strategies of attack angles in acceleration process were summed up. [ABSTRACT FROM AUTHOR]

This research presents a novel method for energy-efficient path planning, aiming to enhance the endurance of unmanned surface vehicle (USV). The proposed method combines the locking sweeping (LS) method, gradient descent method, coastline expansion method, and energy consumption functions. The efficiency of constructing the energy consumption potential map was improved by optimising the LS structure. Moreover, maintaining a user-configurable distance between the USV and the coastline ensures safe paths with minimal energy consumption. The performance of the proposed method was evaluated using multiple simulations involving high-resolution electronic nautical charts and a historical time-variant sea current dataset. The results demonstrate the practicality of the method, and effectively address path planning challenges in a time-variant maritime environment. • Locking sweeping, gradient descent, coastline expanding methods are integrated for USV path planning with complex maritime environments. • High-resolution and complex electronic nautical charts dataset and historic time-varying sea current dataset are used. This feature can improve the practicability of the method. • Adjustable sweep initial point, sweep area and gradient descent step size are proposed as part of locking sweeping method. It can improve the path search efficiency in a time-variant maritime environment. • The proposed method was compared with the three related methods and showed the best performance in energy efficiency. • The research included an analysis of computational time, showcasing its scalability and practicality for real-world applications. [ABSTRACT FROM AUTHOR]

The reference evapotranspiration (ETo) is a key parameter in achieving sustainable use of agricultural water resources. To accurately acquire ETo under limited conditions, this study combined the northern goshawk optimization algorithm (NGO) with the extreme gradient boosting (XGBoost) model to propose a novel NGO-XGBoost model. The performance of this model was evaluated using meteorological data from 30 stations in the North China Plain and compared with XGBoost, random forest (RF), and k nearest neighbor (KNN) models. An ensemble embedded feature selection (EEFS) method combined with the results from RF, XGBoost, adaptive boosting (AdaBoost), and categorical boosting (CatBoost) models is used to obtain the importance of meteorological factors in estimating ETo, and thereby determine the optimal combination of inputs to the model. The results indicated that by using the top 3, 4, and 5 important factors as input combinations, all models achieved high ETo estimation accuracy. It is worth noting that there were significant spatial differences in the estimation precisions of the four models, but the NGO-XGBoost model exhibited consistently high estimation precisions, with global performance indicator (GPI) rankings of 1st, and the range of coefficient of determination (R2), nash efficiency coefficient (NSE), root mean square error (RMSE), mean absolute error (MAE) and mean bias error (MBE) were 0.920–0.998, 0.902–0.998, 0.078–0.623 mm d−1, 0.058–0.430 mm d−1, and −0.254–0.062 mm d−1, respectively. Furthermore, the accuracy of the NGO-XGBoost model in estimating ETo varied across different seasons, which was more significantly affected by humidity and wind speed in winter. When the target station data was insufficient, the NGO-XGBoost model was trained by using the historical data from neighboring stations and still maintained a high precision. Overall, this study recommends a reliable method for estimating ETo, which provides a reference for accurately calculating ETo in the North China Plain in the absence of meteorological data. • Combining four embedded feature selection methods to obtain the importance of factors in estimating ETo. • The importance ranking of meteorological factors in estimating ET 0 were Ra, Tmax, n, RH, u2, Tmean and Tmin. • There were significant seasonal and spatial discrepancies in the ET 0 accuracy of the model in the North China Plain. • The NGO-XGBoost model has higher ETo estimation accuracy compared to other regression models. • The ETo estimation accuracy in spring, fall, and winter in the North China Plain is affected by humidity and wind speed. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a layered inorganic compound—modified zirconium phosphate was used as a framework to assemble fluorescein and rhodamine B dye chromophores. After the dye chromophores were bound to the layered framework, the evident energy transfer process from fluorescein (donor) to rhodamine B (acceptor) was observed, resulting in the dramatic luminescence enhancement for rhodamine B. Without the framework, such energy transfer was absent in aqueous solutions. The results manifest that the lamellar inorganic framework can provide suitable microenvironment to organize the dye chromophores in elaborate arrangements, and promote the intermolecular energy transfer. In addition, the fluorescent lifetime of the donor in different surroundings was analyzed, which further confirmed the energy transfer via a nonradiative process. [Copyright &y& Elsevier]

Lin, J.; Zhao, D.; Guo, C.; Zhang, Z., and Su, Y., 2020. Numerically modeling the effect of flexibility on flow around marine engineering structures: Using the shape of the saguaro cactus. Journal of Coastal Research, 36(3), 628–635. Coconut Creek (Florida), ISSN 0749-0208. The cylinder is an essential infrastructure with a wide range of applications in the engineering fields of shipping, aviation, and construction. The study of the hydrodynamic performance of a cylinder is a current research focus that can provide a more direct reference for the optimized design of its structure. In this study, the principles of bionics and flexible materials are applied to the research of marine structures, and the shape of the saguaro cactus is used as a reference. Considering the influence of flexible materials, a numerical simulation that provides a new route for the research and design of cylinders in the engineering field is performed. Based on the morphological characteristics of the cactus, a three-dimensional model and its fluid-solid control domain are established. Numerical simulations of the flow around rigid and flexible models with different Reynolds numbers are carried out. The lift, drag, flow field characteristics, and structural deformation are obtained. A turbulent vortex street is created by the flow around the cylinder. Vortex-induced vibration is generated, which demonstrates apparent three-dimensional characteristics. In a majority of the cases, flexible materials and irregular cross-sectional shapes have a clear influence on the flow around the cylinder. [ABSTRACT FROM AUTHOR]

Highlights • The bioasphalt exhibited higher rutting resistance than the virgin binder after short-term aging. • Higher rutting resistance of bioasphalt was observed with the addition of crumb rubber particles. • Crumb rubber had an upper limit role on the elastic behavior of binders with the increase of loading frequency. • Rutting factor G*/sin(d) tests was still a valuable tool for the rutting resistance ranking of modified binders. Abstract In this study, the high temperature performances of ten combinations of bioasphalt and crumb rubber particles were evaluated by DSR and MSCR tests. Testing results indicated that short-term aging and crumb rubber modification increased the complex shear modulus and reduced the phase angle. In addition, the crumb rubber modified bioasphalt significantly reduced the unrecovered creep compliance and increased the percentage recovery. It can be concluded that the bioasphalt exhibited higher rutting resistance than the virgin binder and it can be used like traditional binder in term of rutting performance. In addition, crumb rubber greatly increased the rutting performance of bioasphalt. [ABSTRACT FROM AUTHOR]

During voyages, a ship will inevitably encounter wavy conditions that cause heaving motions. These motions induce oscillations in the propeller relative to its surrounding fluid, causing the propeller to operate in off-design conditions under non-uniform inflows. These environments-induced motions will significantly alter propeller performance. To study the motions of the propeller of a surface ship in wavy conditions during actual voyages, numerical simulations were conducted on an oscillating propeller in cavitating flows by using Reynolds-averaged Navier–Stokes (RANS) approach. The rotations and oscillations of the propeller were coupled using an independently defined set of equations of motion, with the oscillations being represented as sinusoidal cyclic motions. The numerical propagation of unsteady flow fields was obtained using the overset grid technique. This approach allowed the hydrodynamic behaviours of a propeller to be fully investigated in the presence of coupling between the rotations and oscillatory motions of the propeller. The effects of non-uniform incoming flows on a propeller's unsteady load and the distribution of sheet cavitation and wake field were analysed for two different loading conditions. The results of these calculations are significant for propeller design and optimization. [ABSTRACT FROM AUTHOR]

This article focuses on the event-triggered model-parameter-free (ETMPF) control problem for autonomous underwater vehicles (AUVs). To realize the trajectory tracking goal under internal model parameter uncertainties and external disturbances, the ETMPF control law is firstly presented via the properties of Euler–Lagrange systems. In the designed ETMPF control law, a static event-triggered mechanism (SETM) is developed to reduce its communication burden with actuators. Then, by introducing internal dynamic variables for the SETM, a dynamic event-triggered mechanism (DETM) is further developed. Compared to the SETM, the maximum triggering frequency under the DETM is computable and can be adjustable by tuning control parameters, which makes it applicable to more serve network resource-limited scenarios. Finally, the effectiveness of the designed two ETMPF control schemes is demonstrated through theoretical analysis and simulation results. • A novel ETMPF control scheme with the SETM is proposed to avoid the dependence on continuous control command with a simpler structure and lower computational burden. • A DETM is further developed to extend the triggering interval and the MIET of the DETM can be strictly positive and computable. • The network bandwidth requirement of the proposed control scheme can be known prior. [ABSTRACT FROM AUTHOR]

This article focuses on the dynamic event-triggered formation control problem for multiple marine surface vehicles (MSVs) considering model uncertainties and external disturbances under a unidirectional communication topology. Firstly, a model-parameter-free control strategy is developed to compensate for the adverse effect of parameter uncertainty and time-varying external disturbances by resorting to the sliding mode philosophy and the adaptive law. In contrast to backstepping-based formation control schemes, the proposed control scheme circumvents the complexity explosion, and there is no complex calculation process. Thus, it has a relatively simple structure. After that, to reduce the communication frequency among inter-MSVs, a dynamic event-triggered control (DETC) mechanism is developed. Compared with the existing event-triggered mechanism, the proposed controller ensures fewer triggering instants among vehicles. Finally, theoretical analysis and numerical simulations have shown the effectiveness and superiorities of the proposed methods. • A novel model-parameter-free control strategy is developed to compensate for the adverse effect of parameter uncertainty and time-varying external disturbances. • A dynamic event-triggered mechanism is designed to reduce network traffic while guaranteeing the system's performance and stability. • The proposed methods possess the properties of high robustness and applicability. [ABSTRACT FROM AUTHOR]

The flow-induced structure noise of underwater vehicles is chiefly caused by the structural vibration under the excitation of turbulent fluctuating pressure, which involves the complex coupling relationship and energy transfer among flow, structure and sound. In this paper, a hybrid numerical method based on computational fluid dynamics (CFD) and computational acoustics (CA) is applied to simulate the flow-induced structure noise of a submerged cone-cylinder-hemisphere combined shell under turbulent excitation. Based on the principle of reverberation method, an experiment was carried out in a low-noise gravity water tunnel, and numerical results were compared with experimental data to validate efficiency and accuracy of the numerical method. It is found that the energy of flow-induced structure noise is mainly concentrated in the medium and low frequency range within 500 Hz. And the line-spectrum characteristics of radiated noise are consistent with the inherent vibration characteristics of the structure. The peak frequencies of the radiated noise spectrum curve correspond to the coupled modes of the structure, and its near-field distribution is greatly affected by the structural vibration characteristics. Fluctuating pressure and flow-induced structure noise characteristics of the cone-cylinder-hemisphere shell under different flow velocities are also compared, which provides meaningful guidance for the noise control of underwater vehicles. • A hybrid numerical method is applied to simulate flow-induced noise of submerged cone-cylinder-hemisphere shell. • Based on the principle of reverberation method, an experiment was carried out in a low-noise gravity water tunnel. • The numerical results were compared with experimental data to validate efficiency and accuracy of the numerical method. [ABSTRACT FROM AUTHOR]

This article focuses on the distributed event-triggered affine formation control problem for multiple underactuated marine surface vehicles (UMSVs) considering affine transformation and communication energy consumption under a directed interaction topology. Firstly, the affine transformation mechanism is adopted to improve the flexibility and maneuverability of the formation. The proposed technique allows a rich collection of collective motions such as translation, rotation, scaling, and the combination of these transformations. Then, unlike the event-triggered control of a single marine vehicle, this paper considers reducing communication frequency between vehicles. Thus, an event-triggered mechanism is proposed, which can save computation resources compared with traditional ones while excluding the Zeno behavior. Subsequently, it is proved that all signals in the closed-loop system are ultimately uniformly bounded (UUB), meanwhile, all tracking errors converge to a small compact set under the proposed control strategy. Finally, numerical simulations illustrate the effectiveness of the proposed formation controller. • A distributed event-triggered affine formation control algorithm is developed to solve the trajectory tracking problem for UMSVs formation. • A novel affine transformation mechanism is devised to enables UMSVs to move as a group in formation translation, shearing, rotation and scaling maneuvers. • An event-triggered scheme is designed to update the measurement signal so as to save the communication resource. [ABSTRACT FROM AUTHOR]

Fish movement investigations typically assume motion through unbound fluids. However, benthic organisms experience ground effects when moving near substrates. To identify the internal ground-effect mechanisms on self-propelled stingrays, simulations applying an in-house developed fluid-structure interaction algorithm were performed. A geometric model was developed from three-dimensional (3D) laser-scan data of a live freshwater stingray. The ratios of distances between the stingray and solid boundary to its disc width and unbounded fluid were investigated at two swimming frequencies: 2.385 and 3.384 Hz. Velocity fluctuation amplitude during one cycle near a substrate was approximately 30% less than that in unbounded fluid. The two peaks of the force coefficient in one cycle decreased and increased, respectively, with decreased power loss near the substrate; this is related to the pressure and wake distributions on the lower stingray surface at two typical instances. Pressure regions on the front and rear lower wing-crest surfaces decreased when approaching the substrate at the first typical instance, whereas that of the wing trough increased for the second typical instance. The vortex size on the lower wing surface decreased near the substrate for both instances. • Stingray self-propelled undulatory motions are simulated near substrates for ground effects. • A geometric model is developed from 3D laser-scanned data of a live freshwater stingray. • Evaluations are performed for four gap distances and unbounded fluid at two swimming frequencies. • Near-substrate velocity fluctuation in one cycle is about 30% less than that in unbounded fluid. • Pressure and wake distributions on the stingray lower surface are noted at two typical instances. [ABSTRACT FROM AUTHOR]

This paper proposes a dynamic event-triggered trajectory tracking control scheme for underactuated marine surface vessels (MSVs) with positive minimum inter-event time (MIET) guarantees. Under the presented scheme, control inputs only need to transmit to actuators at the designed event triggering time. Additionally, the maximum signal transmission frequency can be prior known by designers. To facilitate the design of the above control architecture, an unified second-ordered tracking error dynamic system is firstly derived. Then, based on the transformed error dynamic system, an event-based adaptive control law is developed. To counteract external disturbances and unmodeled dynamics, the minimum-learning-parameter (MLP) based neural approximator is constructed to enhance system robustness. Finally, by introducing internal dynamic variables, a dynamic event-triggered mechanism (DETM) is established to manage communications between the control module and actuators. Compared with existing event-triggered methods, the proposed event-driven method is dynamic and more flexible to satisfy network bandwidth requirements. Besides, MIETs of the presented DETM are also proved to be strictly positive and computable from control parameters. Theoretical analysis and simulation results verify the validity and effectiveness of the proposed DETM based control scheme. • A novel discrete adaptive event-based control law is developed to calculate the event-based control force and moment. • A novel DETM is developed to govern signal transmissions between the control module and actuators with positive MIET guarantees. • The network bandwidth requirement of the proposed control scheme is computable through control parameters. [ABSTRACT FROM AUTHOR]

The purpose of this study was to design a hydrofoil which would improve boat performance through enhanced resistance reduction. Commercial CFD code STARCCM+ was used to solve the Unsteady Reynolds Averaged Navier Stokes Equations for the flow around the boat. Uncertainity study is conducted in order to obtain an effective and reliable numerical calculation method. The method was then validated by direct comparison of the numerical data at different speeds with the test data of USV01 planing boats. Accordingly, twelve hydrofoil design cases were considered, and their resistance reduction performance at 8 m/s was predicted and compared with each other through the numerical calculation method. Effects of hydrofoil parameters such as longitudinal installation position, span, attack angle, installation height on the resistance reduction performance were investigated. One of 12 cases was chosen to investigate the resistance reduction effect of hydrofoil at different speeds. The results show that the hydrofoil, with proper installation position and design parameters, has a significant resistance reduction effect. At 8 m/s, the hydrofoil designed in this paper can reduce boat resistance by up to 30.74%.To analyze the principle of hydrofoil, the flow field around hull and hydrofoil was numerically simulated and studied. • The hydrofoils designed in this paper all had a significant resistance reduction effect at the boat design speed of 8m/s. The maximum reduction of resistance was 30.74% of the total resistance of the bare planing boat. • The current research illustrates that after installing the hydrofoil, the planing boat had a significant resistance reduction effect within the speed range of the real boat speed of 23.33 knots to 38.87 knots. • The hydrofoil could not always achieve the resistance reduction effect in the entire speed range of the planing boat. • The influence of hydrofoil on the flow field around the hull, especially the velocity field, is analyzed numerically. [ABSTRACT FROM AUTHOR]

This paper provides a rotation-matrix-based control scheme for fully actuated autonomous underwater vehicles (AUVs) considering model uncertainties and external disturbances. By resorting to the rotation matrix, AUV's attitude dynamics will be globally and uniquely represented without unwinding. However, this kind of attitude representation approach complicates the controller design process in the sense that the attitude tracking error is defined by a rotation matrix instead of a three-dimension vector. Regarding this, an alternative error vector is introduced for the simplicity of calculation and implementation. Then, a novel sliding mode surface that consists of two components is devised to provide the characters of finite-time convergence and singularity-free. Upon the application of this variable, an artful adaptive law is constructed such that the estimated information always appears as limited values in the controller. Eventually, theoretical analysis and comparative simulations can illustrate the effectiveness of the proposed control scheme. • The rotation matrix is applied for attitude representation of AUVs. • A novel sliding mode surface is proposed for the achievement of the finite-time convergence, singularity avoidance and chattering reduction. • Besides, model uncertainties are handled by a constructive adaptive law. [ABSTRACT FROM AUTHOR]

Abstract: In the paper, graphene acid (GA) was manufactured, using flake graphite as raw material, and the acidity and the structure of GA were characterized as well as. Then, chalcone was synthesized in the presence of GA, using acetophenone and benzaldehyde as the reactant. The results showed that the acidity of GA was for pH = 1.12 in aqueous solution, and it was structured by the graphene sheets with the spaces between the graphene sheet and the graphene sheet and sulfuric acid (H2SO4) and acetic acid (CH3CO2H) inside the spaces. At the same time, the results also exhibited that the chalcone yield was able to reach 60.36% when GA dosage was 5 g, and the chalcone yields could attain apart 60.36, 52.05 and 31.16% when 5 g of GA was used thrice. This shows that GA is not only a high-performance catalyst, but also a controlled-release catalyst. [Copyright &y& Elsevier]

This article investigates the robust finite-time trajectory tracking control problem of under-actuated unmanned surface vessels (USVs) subject to model uncertainties, saturation constraints, and external disturbances. Firstly, the kinematic and dynamic models of under-actuated USVs are transformed into an equivalent tracking error dynamic by resorting to a novel output redefinition-based dynamic transformation (ORDT). Secondly, a smooth dead-zone operator-based model (DOBM) is introduced to deal with the control input saturation constraints problem. On basis of these, a sliding mode-based controller (SMC) is developed, which possesses the properties of chattering-free and finite-time convergence. Later, Lyapunov stability proved that the proposed control strategy is capable of guaranteeing the boundedness of all closed-loop signals, in spite of the parametric uncertainties, external disturbance, and input saturation constraints. Finally, numerical simulations illustrate the effectiveness of the proposed control approach. • A low computation burden control scheme is developed to enhance the real-time performance in identifying uncertain dynamics. • A novel output redefinition-based dynamic transformation method is proposed to achieve a relative degree of underactuated actuator dynamics. • A dead-zone operator-based model with adaptive scheme is proposed to overcomes the nonlinearities induced by unknown input saturation constraints. [ABSTRACT FROM AUTHOR]

This paper investigates the tracking control problem for the marine surface vessel (MSV) under external disturbances, parametric uncertainties, actuator faults, and saturation constraints. The objective is to achieve the approximation-free prescribed performance control with appointed-time convergence. For this purpose, a new appointed-time performance function is designed, and a modified backstepping control design process is conducted. By means of the above contributions, a tracking control scheme possessing the following three features is developed. First, the derived control scheme can characterize the transient and steady-state performance quantitatively a priori. Unlike the conventional prescribed performance control, the appointed-time tracking performance can be guaranteed without the knowledge of initial system conditions. Second, the control design has high robustness against external disturbances and parametric uncertainties. Particularly, the auxiliary compensation system applied for lumped uncertainties is dropped out here such that the control design is simplified. Third, the derived control scheme has both fault-tolerant and anti-saturation merits. Numerical simulations are provided to support the analysis. • A new performance function is designed to obtain the appointed-time tracking performance. Besides, the requirement of the knowledge of exact initial tracking conditions is omitted. • A modified backstepping method is conducted such that the auxiliary compensation system applied for lumped uncertainties can be dropped out. • The derived control scheme has both fault-tolerant and anti-saturation merits. [ABSTRACT FROM AUTHOR]

This paper devotes to tackling the saturated approximation-free tracking control problem for unmanned marine surface vehicles with prescribed performance metrics. Firstly, a smooth dead zone-based model is introduced herein to address the saturation nonlinearity. Then, the tracking error dynamics in presence of saturation constraints and uncertainties are formulated accordingly. After that, the capability to preset the evolution range of tracking errors is guaranteed by means of the prescribed performance technology. It follows that a saturated approximation-free control scheme is developed based on the sliding mode control. Notably, the lumped uncertainties and the saturation nonlinearity are well handled totally free of the online estimations such that the control scheme is simple and intuitiveness for implementation. It is theoretically verified that the controller can achieve the stabilization of the system. Finally, simulation results are presented to illustrate the controller's efficacy. • A model-parameter-free tracking controller is developed for MSVs. • The proposed control scheme can ensure prescribed performance for tracking errors. • The proposed methods possess the properties of high simplicity and intuitiveness. [ABSTRACT FROM AUTHOR]

Stingrays are different from many other aquatic animals; they have dorsoventrally flattened bodies, with enlarged pectoral 'wings' to generate thrust for swimming. In this study, aiming to discover the key features and dynamic response characteristics of stingray swimming, 3D incompressible viscous computational fluid dynamics (CFD) simulations were conducted for self-propelled stingrays. The body geometry was reconstructed based on 3D laser scanner data of a freshwater stingray. The locomotion of the stingray was considered based on the first Fourier mode, and was analyzed using experimental measurements of the 3D kinematics in live stingrays. The amplitude for the body point was obtained using a binary linear function. The swimming velocity, as calculated through the self-propelled simulations, was within 11.9% of the nominal experimental swimming speed. It was found that the Froude efficiency of stingray swimming at slow and fast speeds is approximately 59%, suggesting that the morphological and kinematic characteristics of the stingray allow it to maintain high efficiency during swimming. The wake structures and pressure fields on the dorsal and ventral sides of the stingray were visualized. Moreover, a vortex on the upper surface of the wing's trough and one on the lower surface of the wing's crest were newly discovered. Furthermore, two low-speed and two high-speed boundary frequency cases were numerically simulated. It was found that the average swimming speed during the quasi-steady state has a linear relationship with the frequency of the undulatory motion. The newly discovered vortex on the upper surface of the stingray transforms from scattered small balls into several large balls. This indicates that this vortex plays an important role in lubrication. • Advanced fluid-structure interaction simulation of self-propelled stingray swimming. • Experimental measurement of stingray geometry & motion is used as simulation input. • Bionic stingray can keep high swimming efficiency in a wide range of swimming speed. • Average swimming velocity shows a linear relationship with the undulatory frequency. • Some vortices can play a role of lubrication during swimming. [ABSTRACT FROM AUTHOR]