Aiming at the cluster control problem of small underactuated autonomous underwater vehicles (AUVs), a formation control strategy based on an improved RRT* algorithm is designed.

Paths planned by the RRT* algorithm are steep and difficult to track, with slow convergence speed, so an improved method is proposed to solve the above problems. First, a deviation function is added to bring the random sampling points closer to the target point, then the sampling points are connected smoothly using a Dubins curve. By rerouting within the variable radius range and designing the cost function in relation to the curve length and obstacle avoidance, the best path is chosen. According to the cost and minimum value, multiple AUVs are assigned a rendezvous point, and the speed of multiple AUVs is coordinated to complete the minimum rendezvous time constraint. A segmented vector field construction method based on the Dubins path is then designed, enabling multiple AUVs to track the planned path and reach the target rendezvous point with the direction remaining the same.

The simulation results show that the average path length of multiple AUV formations is shortened by 26.6% and the average assembly time is shortened by 21.7%.

A multiple unmanned surface vehicle (USV) cooperative hunting algorithm based on the double layer switching strategy is proposed to cope with the difficulties of USVs in hunting intelligent escaped targets.

Specifically, the first hunting strategy adopts the improved potential point method. The Hungarian algorithm is employed in order to dynamically allocate potential points for USVs, and the optimization goal is applied to minimize the total linear distance between USVs and potential points. In this process, the artificial potential field method is used to achieve cooperative collision avoidance. The second hunting strategy takes advantage of the nature of the Apollonius circle to tighten the surrounding area, i.e. two USVs go to the target point of the escaped target to intercept it, while the remaining USVs maintain the same direction as the escaped target. Moreover, in order to deal with the different escape strategies of targets, the first and second layers of hunting strategy can be transformed into each other.

Numerical simulation shows that the proposed algorithm can reduce the hunting time to less than or equal to that of the sequential distribution potential point algorithm and polar angle distribution potential point algorithm.

In order to solve the problems of most current ship formation control algorithms such as their long control cycle and poor timeliness, this paper proposes a finite-time control strategy based on an extended state observer (ESO).

First, a nonlinear terminal sliding-mode control algorithm is proposed which overcomes the singularity problem of the traditional terminal sliding-mode by designing the control law in different regions. Then, the nonlinear terminal sliding-mode control and graph theory are then combined to realize finite-time ship formation control. In addition, the ESO is used to observe and compensate for the uncertainties and external disturbances in the ship model in order to ensure the accuracy of ship formation control. Finally, the Lyapunov theorem are used to verify the stability of the ship formation control law.

The simulation results show that the system error of the whole formation approaches zero in about five seconds using the proposed control strategy, thus its achieves stability.

This paper addresses the anti-disturbance optimal coverage control of under-actuated autonomous surface vehicles (ASVs) in fixed ocean areas.

At the kinematic level, the Voronoi tessellation is carried out based on the locations of adjacent ASVs and environmental density information, and guidance laws for under-actuated ASVs are designed to guide each ASV to the optimal target point. Next, considering model uncertainties and external disturbances due to wind, waves and ocean currents, at the kinetic level, a fixed-time extended state observer (FTESO) is developed and a fixed-time kinetic control law is designed on the basis of the FTESO.

Stability analysis shows that the error of the closed-loop anti-disturbance optimal coverage control system is bounded, and simulations are provided to demonstrate the effectiveness of the proposed method.

Aiming at the obstacle avoidance of unmanned ship swarm, this paper proposes an elastic formation control method based on a virtual navigator and improved Hooke's law.

First, the external ellipse is obtained according to the length and width of the obstacle, and the virtual obstacle avoidance area, obstacle avoidance response area and free navigation area are then extended outward according to the aspect ratio in order to simplify obstacle avoidance on the surface of the water. Second, based on the virtual navigator and improved Hooke's law, the constraints on relative distance and speed between each members of the formation as well as those between each members of the formation and the virtual navigator are given, and the formation configuration of the multiple unmanned ships is set in the form of coordinates so as to construct an elastic formation model. Further, based on the obstacle annular repulsive field, the obstacle avoidance of a single unmanned ship is realized. The obstacle avoidance of the multiple unmanned ships can then be realized under the organization of the elastic formation. Moreover, the sailing process of unmanned ship is optimized to eliminate the phenomena of "wandering" and "shaking" in the process of formation and obstacle avoidance, and increase the smoothness of the movement of the unmanned ship. Finally, four unmanned ships are used to form a formation, and a simulation experiment based on Matlab is carried out.

The results show that all obstacles are successfully bypassed, verifying the effectiveness and feasibility of the proposed method.

This paper studies the hydrodynamic characteristics of submersible under the interference of seabed sand wave topography.

Based on the computational fluid dynamics (CFD) method, the process of submersible passing above a seabed sand wave is simulated in the STAR-CCM+ software using the overset grid method, and the evolution of the resistance coefficient and lift coefficient during the process is calculated.

The results show that when the submersible navigates near the seabed, interaction forces act on it. When the bow moves near the wave crest, the maximum negative lift occurs, and when the stern moves near the crest, the maximum resistance occurs. The different surface pressure distribution rules of submersible at different speeds lead to different evolution rules of the lift coefficient. The smaller the H/D, the more significant the influence of the seabed sand wave on the resistance and lift force of the submersible. When H/D > 5, the influence of the seabed sand wave on the resistance and lift force of the submersible can be ignored.

This study seeks to improve the thermal management technology of electronic devices through constructal optimization design.

First, the 3D flow and heat transfer numerical model of a hybrid microchannel heat sink is established, and the volume ratio and relative position of its single-sided internal fins are fixed. Constructal optimization is then performed, for which the height-to-width ratio h/w and spacing distribution of fins ε are taken as the design variables, and the minimization of the temperature gradient uniformity factor in the solid region of the heat sink is taken as the objective. The exhaustive method and genetic algorithm are subsequently performed.

The results show that both the exhaustive method and genetic algorithm can find the minimum value of the temperature gradient uniformity factor and corresponding optimal construct, and the temperature gradient uniformity is improved by up to 13.30% compared to that of the finless microchannel heat sink. The relative difference in the obtained minimum objective function value between the exhaustive method and ten genetic algorithm schemes is no more than 0.92%.

This paper aims to study the problems that the external electrical characteristics of a hydrogen fuel cell are soft, its dynamic characteristics are poor and its system stability is susceptible to the influence of propulsion load in marine hydrogen storage DC electric propulsion systems.

First, an analysis is performed of the output external electrical characteristics of the hydrogen fuel cell and the propeller load conditions of the marine electric propulsion system, then a ship-engine-propeller model and a frequency-domain model of drive control system for a permanent magnet synchronous motor (PMSM) are set up. Next, a speed-loop bandwidth design method is proposed, considering the external electrical characteristics of hydrogen fuel cell and propeller load conditions. Finally, on basis of the parameters of a mother ship, an electric propulsion system for a hydrogen-battery DC electric propulsion ship in a hardware-in-loop experimental platform is established to verify the proposed method.

The experimental results show that the speed response of the motor has no overshoot under this method, and the speed-loop fluctuation is reduced by 5 r/min when the load torque disturbance occurs.

In this paper, experiments are carried out to investigate the effects of water spray on the soot removal of diesel engine flue gas.

A Dekati low pressure impactor (DLPI) is used to sample smoke soot in a flue. The sampling gun is wrapped in a heating belt and the temperature is controlled to prevent the condensation of wet flue gas after spraying and maintain the same state of soot in the flue. First, the uniformity of flue gas concentration is measured by a flue gas analyzer to verify that the flue gas distribution is basically uniform, and the sampling time is determined by DLPI. The soot purification efficiency under different diesel engine loads and different water spray volumes is then measured and compared.

The results show that the efficiency of soot removal can reach 50%. Under the same spray rate, the soot removal efficiency of the diesel engine under 100% load is higher than that under 50% load. When the spray rate reaches a certain value, the efficiency of soot collection and purification may reach the extreme value. Under 50% load, the contact time between water spray and soot is longer, and the agglomeration effect of small particles is more obvious than under 100% load.

To investigate ship motion and load responses in realistic 3D waves and overcome the limitations of the traditional 2D wave assumption, this paper develops a method for predicting ship motion and load responses in short-crested waves.

The long- and short-term responses of ship motion and load in long- and short-crested waves are numerically predicted using the spectral analysis and statistical probability methods, respectively. The influence of directional function on ship response is also numerically analyzed. Moreover, tank model tests and a large-scale model sea trial are comparatively conducted to validate the difference between ship response and statistics in long- and short-crested irregular waves.

The results show that when navigating against the waves in the same sea condition, the long-crested wave assumption overestimates the statistical mean value of ship load response, but underestimates extreme load in real seas. For long-crested waves, the ship motion and acceleration power spectrum is concentrated around a certain frequency band.

A robust adaptive course-keeping control algorithm is designed to deal with the course-keeping problem for under-actuated ships with rudder faults, gain uncertainty and marine disturbances.

By combining the robust neural damping technique and adaptive approach, numerous neural network (NN) weights can be compressed horizontally, and only two gain-related adaptive learning parameters need to be designed to compensate for both the gain uncertainty and unknown fault parameters. The proposed controller is proven to be semi-global uniform and ultimately bounded (SGUUB) through Lyapunov analysis. Finally, the Nomoto mathematical model is established using "Yukun", and the effectiveness and superiority of the course-keeping algorithm is illustrated by carrying out comparison experiments under marine interference conditions.

The results show that the average rudder angle of "Yukun" under rudder failure is reduced by 51%, significantly improving control performance.

Aiming at the problem of poor depth-changing ability of the stern rudder of submarine at low speed, and the problem of reversing the movement trend is difficult when the stern rudder is stuck and encountering abrupt changes of seawater density in the vertical steering maneuver. This paper conducts the mechanism research of static moment maneuver.

First, it is proved through theoretical analysis that the static moment maneuver can eliminate the reversed velocity phenomenon of the stern rudder. Then, the surfacing process under the static moment maneuver and the rudder hydrodynamic control is numerically simulated.

The simulation results show that the static moment maneuver can make the submarine maintain good vertical plane maneuverability at low speed, eliminate the reversed velocity phenomenon of the stern rudder, and avoid excessive trim angle of the submarine at high speed. At the same time, the recovery effect of the two emergency situations of rudder jamming and falling deep has been improved.

This paper aims to suppress the adverse effects of tip clearance flow on the hydrodynamic performance and unsteady excitation force of a pump-jet propulsor.

As for the pre-swirl stator pump-jet propulsor, an annular flexible seal structure closely matched with a rotor tip ring is used to study the validity of suppressing the clearance flow on the rotor tip. The rotor thrust and torque of the propulsor are measured by keeping the shroud approximately rigidly fixed, enabling the rotor open water efficiency to be obtained. In addition, the point of cavitation inception at each design condition is observed and recorded carefully with the help of a high-power stroboscope, and the cavitation inception curves of the propulsor with/without tip clearance are obtained through calculation. Finally, tests of shaft vibration acceleration on the pump-jet propulsor with/without tip clearance are conducted under conditions of cavitation and non-cavitation in order to evaluate the effects of diminishing tip clearance.

The results show that the thrust and torque of the rotor of the pump-jet propulsor with a flexible seal structure are significantly increased, which in turn renders open water efficiency significantly increased at low and medium advance coefficients, unchanged near the design point and slightly decreased at the high advance coefficient. Moreover, the cavitation performance of the pump-jet propulsor without clearance is better at a wide range of advance coefficient, 0.85 <J< 1.40, near the design point. In addition, compared with a pump-jet propulsor with clearance, the amplitude of axial vibration acceleration of the shaft equipped with a pump-jet propulsor without tip clearance is significantly reduced at most characteristic frequencies.

Infrared radiative (IR) characteristics of the plume are the main part of ship infrared radiation. The influence analysis of exhaust parameters on radiative characteristics is very important for developing ship stealth and anti-stealth technology.

In this work, the line-by-line method is applied to solve radiative transfer in gaseous media. Infrared radiative images of plumes under different temperatures and flow rates are obtained. Radiance, spectral radiative intensity, and spectrum-integrated radiative intensity over 3-5 μm are calculated and analyzed.

Results show that there are obvious peaks of spectral radiative intensity curves at 4.18μm and 4.43 μm, and an obvious valley at 4.26 μm.

This paper aims to evaluate the impact of flow-induced vibration on the receiver performance of low-frequency magnetic antenna in an underwater environment, investigating the electromagnetic-induced noise of antenna under different vibration conditions.

First, a fluid excitation antenna vibration model is established using COMSOL software, then an antenna vibration-induced noise model is constructed to simulate electromagnetic-induced noise. Finally, outdoor experiments to measure the antenna vibration-induced noise are carried out.

The results show that the stronger the antenna vibration is, the greater the electromagnetic-induced noise is. When the antenna vibration acceleration reaches 0.029 4 m/s², the electromagnetic-induced noise above the pT level will be caused, and the normal communication signal will be submerged.

In order to clarify the interference of ship movement on the radar cross section (RCS) measurement statistics of a real ship, the statistical characteristics of ship body X-band RCS under various motion states are analyzed.

A dynamic RCS simulation method of a ship body under low sea state motion is established, and a unified model of hydrodynamic simulation and electromagnetic scattering is created on the basis of the DTMB 5415 benchmark surface ship model. The dynamic RCS data of ships detected by X-band radar at a horizontal incident angle are obtained, and the influence boundaries and rules of statistical time, sea state, speed and sea wave direction on the statistical features of RCS are analyzed.

The results show that the statistical characteristics of the dynamic RCS of a ship body are different from those in a static state. The influence range of the general values of RCS under low sea state, speed and wave angle is within 0.9 dB. The RCS of a ship's characteristic direction is sensitive to changes in the sea wave direction angle, and decreases gradually with the increase of the sea state.

This paper aims to study and optimize the influence of Magnus airfoil and variable angle flaps on the aerodynamic performance of unmanned sailboats in order to improve the sailing efficiency.

NACA 0021 is used as the basic airfoil for the mainsail, and the Magnus cylinder is coupled with the tip edge of the mainsail to analyze the influence of key parameters (i.e., diameter, position and gap) on the lift-drag characteristics of the airfoil. On this basis, a flap sail is embedded at the trailing edge of the mainsail to study the flow field conditions and lift-drag characteristics of the airfoil under different flap deflection angles, and its influence on the thrust performance of the unmanned sailboat.

The results show that under a large angle of attack, the Magnus cylinder has an improving effect on the aerodynamic performance of the airfoil; its lift-to-drag ratio is positively correlated with position and negatively correlated with diameter and gap within the studied range; diameter and gap have a greater effect on the aerodynamic performance of the airfoil than the position; the variable angle flap has a more obvious improving effect on the aerodynamic performance of the airfoil under a small angle of attack, and the lift-to-drag ratio is positively correlated with the flap deflection angle in a 0°–15° angle of attack; and within the studied range, the joint action of the Magnus cylinder and flap has a maximum improvement of 27% on the thrust coefficient of the sail, and the thrust coefficient is positively correlated with the flap deflection angle.

A rotational cavitator generates supercavitation through the high-speed rotation of its blades in water to meet the practical application requirements of various projects. Therefore, it is necessary to optimize the blade profile in order to improve the working performance, and investigate the influence of the improvement of the blade profile on the hydrodynamic characteristics of the cavitator.

First, the original wedge-shaped blade is modified, and 3D geometric models of the cavitator before and after the modification are established. Numerical simulations are then carried out of the natural cavitation generated by two cavitators at different rotational speeds. Finally, the hydrodynamic characteristics of the two cavitators are compared and analyzed according to the calculation results.

The results show that with the original blade profile, the cavity is only generated by the trailing edge, but with the improved blade profile, the cavity can be also generated by the secondary leading edge, and the two cavities are gradually connected with increasing rotational speed. Therefore, the rotational cavitator with the improved blade profile produces a larger cavity and stronger natural cavitation. In addition, with the improved blade profile, the cavitation effect is relatively stronger at the blade root, while with the original blade profile, it is stronger at the blade tip. The cavity generated by the cavitator with the improved blade profile at higher rotational speed touches the surrounding wall of the device, resulting in a linear change in the shape of the cavity tail along the radius.

This paper aims to study the effects of the freezing phenomenon of supercooled water droplets in intake air on the aerodynamic performance of a marine power intake system under cold marine environment conditions.

First, an NACA 0012 airfoil is taken as the research object, its aerodynamic performance is numerically simulated and the validation of the established model and numerical simulation method are verified through comparison with the experimental results. Commercial software Fluent is then further developed using the user-defined function (UDF), the numerical simulation of the impingement characteristics of supercooled water droplets is carried out based on the Lagrange method, and the water collection coefficients corresponding to various inflow conditions are obtained. Finally, combined with dynamic mesh technology and the user-defined function, the icing characteristics of the airfoil and its aerodynamic performance degradation characteristics after icing are numerically studied.

The results show that the intake flow angle and diameter of water droplets have a significant influence on the impingement range of the droplets and the water collection coefficient, while incoming velocity has little effect on the impingement range of the droplets, but a certain influence on the water collection coefficient. In addition, under the condition of zero degree angle of attack, the ice accretion on the leading edge of the airfoil has little effect on its aerodynamic performance. Conversely, under the condition of five degrees angle of attack, the icing of the leading edge has a serious impact on its aerodynamic performance.

In order to solve the problem of numerical simulation of flat-nosed projectile penetration into metal plates, the influence of mesh size on element failure strain value and residual velocity of projectiles was studied.

The finite element software LS-DYNA was used to simulate the process of uniaxial tensile test of Q235 steel sample, and the failure strain of the element under the grid density is obtained by the elongation of the tensile sample during fracture. In the meantime, the correction curve of the failure strain with the grid density was plotted and dynamically corrected. Then, the numerical simulation of flat-nosed projectile penetrating Q235 steel plate was carried out with the target plate meshed with different sizes. The failure strain of Q235 steel material was selected according to the correction curve. Finally, the residual velocity of the projectile is compared with the experimental results to analyze the influence of mesh size on the simulation results of the penetration resistance problem of the metal plate in the numerical simulation.

The results show that the element failure strain selected in the numerical simulation should increase with the increase in grid density, and in the case of the metal plate anti-penetration problem, it should increase with the increase of the grid density. In the problem of penetration resistance of metal plates, the simulation results of residual velocity prediction gradually converge with the experimental results. With the increase of mesh density, when the grid size is 0.5 mm, the average relative error of the numerical simulation and the test fitting curve in the velocity section is 5.13%, and the error between the numerical simulation and the test is larger in the low-speed section. Moreover, the residual velocity of the projectile body is more sensitive to mesh density in the low velocity range.

During the mission, the ship may be simultaneously subjected to the combined wave load and underwater explosion bubble pulsation load, resulting in the "superposition effect" of hull response and the loss of total strength of the ship. Therefore, it is necessary to explore the dynamic response law of hull girder under the combined action of above-mentioned loads.

First, a simplified hull girder model is established by theoretical analysis, and the bubble pulsation load of underwater explosion and wave load are solved. Then, based on the Hamilton Principle, the differential equations of motion of hull girder at both free ends subjected to the two loads are derived separately and jointly. Finally, based on the solution of the motion differential equation, the hull girder's free vibration response and the simplified model's motion response under the three working conditions of external load combination are analyzed.

The results show that under the combined action of the two loads, the motion response of the hull girder is 15% larger than the linear superposition of the motion response of the two loads alone.

In order to explore the relationship and difference between the intrinsic dynamic characteristics of the orthogonally stiffened plate of different dimensions, the scale effect of the intrinsic dynamic characteristics of the construction members, such as the beam and the plate, as well as the dynamically coupled structure of the two construction members, the orthogonally stiffened plate, was analyzed and discussed.

Taking the models of the beam, the plate and the orthogonally stiffened plate as research objects, the natural frequencies and mode shapes of the above-mentioned models of different sizes were calculated by the finite element method. The influence of boundary conditions on the scale effect is discussed in terms of the similarities and differences between the natural frequencies of the beam model and the plate model of the unit scale under the free boundary conditions with those of the models under the simply supported boundary conditions. The influence of dynamic coupling between the construction members of the beam and the plate on the size effect of the intrinsic dynamic characteristics of the orthogonally stiffened plate was investigated by discussing the similarities and differences between the natural frequencies and mode shapes of the orthogonally stiffened plate of unit scale with those of the beam model and the plate model of unit scale.

Numerical results show that the variation of the natural frequencies of the model of unit scale against the size of the model under free boundary conditions no longer strictly follows the law that the natural frequencies are inversely proportional to the square of the size, but it is generally similar, or close to, the law.

This paper studies the influence of impact load on the dynamic response of marine stiffened plates.

According to the relevant specifications and references, the sizes of hull stiffened plates are determined. Finite element software ANSYS is used for modeling with Shell 181 element. The load is applied uniformly on the surface of the stiffened plate, and its boundary condition is fixed. The impacts of the natural frequency and relative stiffness of the stiffened plate on its nonlinear dynamic response under impact load are then analyzed.

The results show that the piecewise function relations expression of natural frequency and deflection response are obtained according to the different influences of geometric dimensions. The relative stiffness of the stiffened plate affects its deformation mode, and the critical values are 0.2 and 19.

Due to the larger length-to-diameter ratio of the stern bearing, it is difficult to reflect its actual operating conditions when simplified to the traditional equivalent model of single-point support. Therefore, the influence of the equivalent form of the stern bearing on the transverse vibration characteristics of the shafting is investigated.

The improved Fourier series is introduced to describe the lateral vibration displacement of propulsion shafting. Then, the calculation model of lateral vibration performance of propulsion shafting under various equivalent forms, such as single-point support, multi-point support or continuous distributed support, are constructed based on the energy principle. Thereby, the influence of the change of support stiffness equivalent to the liquid film pressure on the lateral vibration of the shafting and the influence of the propeller excitation on the vibration response of the shafting are further analyzed. Finally, the results acquired by the proposed model is compared with the results of related references and finite element method (FEM) to verify the validity of the calculation model.

The multi-point support calculation results converge to the continuous distributed support calculation results. The three-point support equivalent form can be used to study the influence of liquid film pressure distribution on the lateral vibration characteristics of the propulsion shafting. The shafting response under propeller excitation is affected by the revolution speed.

In order to reconstruct the 3D shape of the non-structural surface of the hull plate frame accurately and quickly, and lay a foundation for the study on the high-precision and efficient measurement of the hull structure deformation, a model reconstruction method based on RGB-D depth image was designed.

First, the Random Sampling Consistency algorithm and the Least Square Method are combined to remove outliers in the point cloud, and then the checkerboard target position information of RGB color image is used to register the Multi-View Cloud of structure. Second, the point cloud is clustered by the regional grid of the structure surface, and the spatial curved surface of each grid point cloud subset is fitted by using the Least Square Method to realize point cloud fusion. On this basis, the high-order panel element is used to realize the 3D reconstruction of the hull structure outer plate surface. Finally, the accuracy of the model reconstruction method is verified by comparing the specimen reconstruction model with the laser scanning point cloud.

The results show that compared with the laser scanning point cloud, the root mean square error of the random points on the 3D reconstruction model of the test object is 1.02 mm. The modeling accuracy meets the needs of ship construction engineering, but the data acquisition time of the structure RGB-D depth image is negligible compared with the laser scanning.

Due to the different arrangement and control mode of an underwater vehicle with an X-rudder configuration, the effectiveness of the after-control-surface is influenced by the angle of incidence of the underwater vehicle.

Numerical simulation is carried out on SUBOFF model with an X-rudder configuration, including variations of single rudder coupled with attack angle and drift angle respectively, along with fitting research for each case.

The results show that the hydrodynamic characteristics related to rudder angle for the two orthogonal planes are different because of the significant interaction between the rudder and main body. The effectiveness of the top right rudder and lower left rudder decreases and increases with the incidence angle respectively for the studied range of incidence and rudder angle, and is also different from its straight running conditions. The relative change of the rudder derivative is up to 16% in quantity.