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This article studies the distributed formation control problem for multiple unmanned surface vehicles (USVs) considering uncertain coefficient matrixes, unmeasurable velocities, and time-varying disturbances. The main contributions are as follows: First, a global coordinate translation is proposed to partially linearize the nonlinear dynamic model equipped with the unmeasurable velocity. Second, based on the global coordinate translation, a novel type of fixed-time extended two-state observer (FTETSO) is developed to estimate unmeasurable velocities and total disturbances for each vehicle. Wherein, the estimation errors will converge to zero within a fixed time. Meanwhile, considering estimation accuracy, a two-state extension is proposed to replace a single-state extension. Third, using a sliding model-based control technique, an FTETSO-based distributed global output-feedback fixed-time formation controller (GOFFC) is elaborately developed. Based on the proposed controller, the fixed-time convergence of the closed-loop system is ensured. Finally, the validity and stability of the proposed control approach are verified by simulations. • A newly introduced decoupling technique can guarantee global stability. • A novel fixed-time extended two-state observer is proposed. • A distributed global output-feedback fixed-time formation control scheme is proposed. • The observation errors and tracking errors converge to zero in a fixed time. [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]

This paper provides a model-parameter-free control strategy for the trajectory tracking problem of the autonomous underwater vehicle exposed to external disturbances and actuator failures. Two control architectures have been constructed such that the system states could be forced to the desired trajectories with acceptable performance. By combining sliding mode control (SMC) technology and adaptive algorithm, the first control architecture is developed for tracking missions under healthy actuators. Taking actuator failures scenario into account, system reliability is improved considerably by the utilization of a passive fault-tolerant technology in the second controller. Benefitting from properties of Euler–Lagrange systems, the nonlinear dynamics of the underwater vehicles could be handled properly such that the proposed controllers could be developed without model parameters. Finally, the validity of the proposed controllers is demonstrated by theoretical analysis and numerical simulations. • Two model-parameter-free tracking controllers are developed for AUVs. • With proper transformation, actuator faults could be accommodated with satisfactory performance. • The proposed methods possess the properties of high simplicity and intuitiveness. [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]

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]

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]

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]

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]

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]

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]

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]

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]

This paper analyzes the characteristics of the wake dynamics of a propeller operating before a rudder under open water conditions. The analysis is conducted using a numerical method based on the detached eddy simulation (DES). In particular, the evolution mechanism of the propeller wake is explored under different advance coefficient and rudder angle conditions. Under heavy load conditions, the topology of the vortex system becomes more complex and is accompanied by a stronger interaction between the wake and the rudder. In addition, a secondary vortical structure bridging the tip vortices is formed in the vicinity of the tip vortex, followed by the growth of a secondary vortex and the formation of a secondary vortex pair. At nonzero rudder angles, the interaction between the tip vortex and the rudder becomes stronger, yielding a more complex vortex topology. Furthermore, an intertwined vortical structure is formed on the suction side and the Ω -shaped vortical structures form much earlier. Analysis of the kinetic energy (KE) and pressure spectra on the rudder surface provides additional insights into the evolution pattern of the vortical structures. The objective of this study is to further identify the dynamic characteristics of the propeller vortices before the rudder under multiple operating conditions. • The wake dynamics of a propeller operating before a rudder is investigated based on DES method. • The evolution mechanism of the propeller wake is detailed under various working conditions. • The vortex system becomes very complex under high loading and finite angle conditions. • The viscous interaction between the tip vortex and the boundary layer of the rudder is investigated. [ABSTRACT FROM AUTHOR]

• A bow hydrofoil is designed to improve the seakeeping of the planing craft in regular waves at the high speed. • The seakeeping performance of planing craft is predicted, and the numerical method is verified reliable. • The influences of installation height and attack angle are analyzed and the suitable parameters are selected. • The influence of the speed on the effect of the hydrofoil is analyzed through comparison with the bare vessel. This paper analyzes the hydrodynamic performance of a planing craft with a fixed hydrofoil in regular waves. Numerical simulations are carried out based on a RANS-VOF solver to study the hydrodynamic performance of the planing craft and the influence of the fixed hydrofoil on its seakeeping. To validate the numerical method, a series of hydrodynamic experiments of a bare planing craft without the hydrofoil were carried out, from which the seakeeping performance of the planing craft was recorded, the numerical method based on overset grid was compared with the experiment and verified reliable. Eight hydrofoil design cases were then studied, whereby, their seakeeping performance in regular wave conditions were predicted through the numerical method which has been verified reliable and compared with each other. Effects of hydrofoil parameters, such as angle of attack and installation height, on the seakeeping performance were investigated. Finally, the suitable installation parameters which can optimize the performance of hydrofoil and reduce the negative influence are verified. The influence of the speed on the effect of the hydrofoil and the flow field around the planing craft are also investigated. [ABSTRACT FROM AUTHOR]