## 2021 Vol. 16, No. 6

2021, 16(6): 1-2.
Abstract:
2021, 16(6): 1-9, 33. doi: 10.19693/j.issn.1673-3185.02092
Abstract:
Objectives  In order to realize the formation control of swarms of unmanned marine vehicle (UMV) with directed interaction topology, the formation tracking control of unmanned surface vehicle (USV) and unmanned aerial vehicle (UAV) swarms is studied.  Methods   An inner/outer loop layered control structure of formation is adopted to model the system, and the discrete-time system is used to describe the formation control problem of UMV swarms, improving the compatibility and practicability of the method for heterogeneous swarms. Based on the idea of a virtual leader, a distributed formation control protocol based on neighbor position and velocity error feedback is adopted, and the necessary and sufficient conditions for UMV swarms to realize the desired configuration and path tracking are given.  Results  The simulation results show that UMV swarms can achieve stable formation control when there is a directed spanning tree in the interaction topology, and the parameters of the control protocol and discrete time period meet the given constraints.  Conclusions  This method has application value in the formation control of UMV swarms with directed interaction topology.
2021, 16(6): 10-18. doi: 10.19693/j.issn.1673-3185.02095
Abstract:
Objectives   The problem of autonomous obstacle avoidance of unmanned surface vehicles (USV) under dense obstacle sea conditions is studied.   Methods   An improved dual-window dynamic window approach (DWA) obstacle avoidance algorithm based on a fuzzy control strategy is proposed; that is, a sensing window based on an onboard sensor is designed on the basis of the conventional speed window, and a dual-window model composed of the speed window and sensing window is further optimized. The speed space is constrained and the weight of the evaluation function is dynamically adjusted based on the fuzzy control strategy in accordance with the obstacle distribution state and distance between the USV and obstacles.   Results   The simulation experimental results show that, compared with the original DWA algorithm, the path planned by the improved algorithm under unknown dense obstacle sea conditions is smoother and more reasonable, which not only solves the problem of USVs detouring outside dense obstacle groups, but also improves the safety of obstacle avoidance navigation and reduces the number of iterations and running time by more than 20%.   Conclusions   The results of this study have certain reference value for research on the autonomous obstacle avoidance technology of USVs.
2021, 16(6): 19-26, 60. doi: 10.19693/j.issn.1673-3185.02057
Abstract:
Objective  Sailling saftey is the chief matter for ship navigation, supposing that collision avoidance operation is heavily dependent on the captain's performance or judgement, it would pose potential risks to the ship safety. In order to coordinate all ships (tourist ships, cargo ships, etc.) in key waters and predict their routes, it is necessary to establish an anti-collision mechanism.  Methods  Using the deep deterministic policy gradient (DDPG) algorithm and the Fujii's ship domain model, an electronic chart is used to simulate the ship's navigation route, and an improved strategy for the DDPG algorithm based on the key learning of failure regions and the improved parameters of the ship domain model according to the characteristics of tourist ships are proposed, so as to enhance the accuracy of route prediction and anti-collision.  Results  Using the improved DDPG algorithm and ship domain model, compared with the previous algorithm, the accuracy of ship collision avoidance is raised from 84.9% to 89.7%, and the average error between the simulated and real route is reduced from 25.2 m to 21.4 m.  Conclusion  Through the proposed ship collision avoidance path planning based on the improved DDPG algorithm and ship domain model, the supervision function of ship routes in water areas can be realized; when the predicted route intersects with other ships, the dispatcher will be alerted, realizing an effective anti-collision early warning mechanism.
2021, 16(6): 27-33. doi: 10.19693/j.issn.1673-3185.02022
Abstract:
Objectives  A multi-constraint two-dimensional packing algorithm is used to determine the carrier-based aircraft maximum density layout factor.  Methods  First, the constraints are presented. Then, based on a lowest-gravity-center NFP algorithm combined with mathematics modeling for the distance constraints and a heuristic algorithm for the "keep-to-the-boundary" spotting, an entire maximum density layout algorithm for a flight and hanger deck is presented.  Results  Using this algorithm, the maximum layout number of F/A-18C and F-35C aircraft on a Nimitz-class aircraft carrier is determined, as well as the layout factor of F-35C aircraft, and the results are consistent with the known facts.  Conclusions  With this algorithm, the maximum density layout factor of aircraft can be quickly calculated, making it useful for guiding the suitable layout design of newly-built carrier-based aircraft.
2021, 16(6): 34-44. doi: 10.19693/j.issn.1673-3185.02019
Abstract:
Objectives   Flight-deck support operations are the most critical link in the process of launch and recovery of carrier-based aircraft.The reasonable allocation of support personnel can enable to improve the capability of supporting the aircraft fleet operation and the sortie generation rate, thereby enhancing the overall combat effectiveness of the carrier.  Method   Aiming at the personnel configuration problem of the carrier-based aircraft flight-deck operation support, considering the complex realistic environment and based on the makespan of the aircraft fleet operation support, load balance and cumulative transfer time of support personnel on flight deck, a multi-objective mathematical programming model is established to obtain the required numbers of support personnel and skill allocation scheme. An integer encoding method and decoding method based on the event-based scheduling policy are designed, and an improved NSGA2-based personnel optimization algorithm is proposed to solve the problem.  Results   The simulation results show that this algorithm can effectively solve the established mathematical model, and the simulation results meet the actual combat requirements.   Conclusion   The improved NSGA2 algorithm can combine the carrier-based aircraft support process, numbers of personnel and skill allocation to provide the carrier-based aircraft support operation with a multi-objective optimized scheduling plan.
2021, 16(6): 45-51. doi: 10.19693/j.issn.1673-3185.02129
Abstract:
Objectives  Naval ship systems such as the hull structure, weapons equipment and power equipment will deteriorate during their service life. Thus, a ship maintenance strategy based on the actual deterioration state is essential for ensuring the safety and availability of naval ships.   Methods  In this paper, a multi-state deterioration system model is established on the basis of the Markov decision process. A reinforcement learning mode is then introduced to train the agent that generates the maintenance strategy, and the optimal condition-based maintenance strategy is obtained in the process of adaptive learning.   Results  The proposed method is applied to a ship structural deterioration system for demonstration, and the results show that it can obtain the optimal maintenance policy for a multi-state deterioration system considering the actual conditions, thereby providing an intelligent supporting tool for decision-makers to formulate optimal ship maintenance strategies.   Conclusions  This paper shows that the reinforcement learning method has great potential in comprehensively improving ship maintenance support.
2021, 16(6): 52-60. doi: 10.19693/j.issn.1673-3185.02333
Abstract:
Objective  To establish a reliability, maintainability, supportability (RMS) assessment model for shipboard radar from macro and micro perspectives, find the weak links that restrict the enhancement of radar effectiveness, and improve the mission success of shipboard radar, this paper proposes the general approach of a multi-agent-based RMS simulation assessment model for shipboard radar.  Methods  Through an analysis of the radar operation status and internal structure, a mathematical model of "mission-equipment-support resources" RMS assessment is established and mapped into different agent types. Based on multi-agent technology, the dynamic changes of equipment operation status, maintenance support mechanisms, and assessment index output are modeled, and the multi-agent-based RMS simulation assessment system is established. Combined with typical examples, the RMS evaluation modeling of a type of phased radar antenna array is carried out.  Results  The simulation obtains the mission success rate and RMS-related assessment parameters of the antenna array, which verify the feasibility of the multi-agent-based radar simulation assessment method.  Conclusion  The results of this study lay the foundation for the scientific RMS design improvement of shipboard radar equipment, and provide references for research regarding other shipboard equipment.
2021, 16(6): 61-71. doi: 10.19693/j.issn.1673-3185.02337
Abstract:
Objective  Aiming at the problem that existing ship combat readiness assessment methods cannot meet the needs of naval mission support, a ship combat readiness assessment method based on cloud model theory is proposed.  Methods  First, in the process of determining the index, based on the cooperative game weight method, the weights calculated by the analytic hierarchy process, entropy weight method and grey correlation method are used for the cooperative game in order to fit the combined fixed weight, and variable weight theory is introduced to modify and optimize the fixed weight. The cloud model theory is then introduced, and a fuzzy comprehensive assessment model based on the cloud model is designed using cloud similarity instead of membership degree. Finally, taking air defense tasks as an example, the combat readiness of the ship is assessed.   Results  The simulation results show that under the variable weight mode, the fuzzy comprehensive evaluation results based on the cloud model can more accurately reflect the combat readiness of real ships.   Conclusion  The results of this study can provide references for ship combat readiness assessment.
2021, 16(6): 72-76. doi: 10.19693/j.issn.1673-3185.02294
Abstract:
Objectives  In response to the new requirements for the accurate forecasting of ship planned maintenance costs, a forecasting method via case-based reasoning is proposed.   Methods  First, the feature vectors composed of the main feature attributes of various types of ships and their maintenance costs are represented by cases. The K-nearest neighbor (KNN) algorithm based on weighted Euclidean distance is then used for case retrieval, and the attribute importance of rough set theory is introduced. Second, the similarity between the retrieved case and target case is used as the adjustment coefficient, and each case is revised in combination with the idea of combined forecasting. Finally, the latest case obtained from the forecasting is added to the case library for the continuous accumulation of data.   Results  The comparative analysis results of this method, the linear regression forecasting method and the radial basis function (RBF) neural network method against real ship maintenance data show that the average forecasting relative errors are 8.7%, 10.4% and 10.2%, verifying this method's forecasting accuracy and validity.  Conclusion  The results of this study can provide references for the formulation and optimization of ship maintenance cost plans.
2021, 16(6): 77-83. doi: 10.19693/j.issn.1673-3185.02298
Abstract:
Objectives  The "cost plus 5% profit" pricing method has been adopted for ship equipment scheduled repair for many years, but it does not reflect quality and schedule of repair project, so it is unable to motivate contractors to cost-effectively improve their repair work.  Methods  This paper chooses cost, schedule and quality as the incentive factors, and develops an incentive fee method which distinguishes between the fixed and incentive parts of the cost, with a schedule-based incentive fee to encourage the elimination of delays by Party A and a quality-based incentive fee to encourage the construction of a quality evaluation index based on the features of ship equipment scheduled repair.  Results  The example analyses show that the contractors can thus earn reasonable profits from contracts adopting this method, Among the examples,projects with better overall performance can break the 5% profit margin limit.  Conclusions   The method put forward in this paper can effectively motivate contractors to control their costs, improve their quality and accelerate their schedule. This is of great practical significance for perfecting the incentive fee mechanism and promoting effective ship equipment scheduled repair.
2021, 16(6): 84-91, 150. doi: 10.19693/j.issn.1673-3185.02104
Abstract:
This paper briefly introduces particle image velocimetry (PIV) technology and summarizes the R&D of algorithms for velocity field calculation in the PIV image treatment. More attentions are paid to various applications of PIV in ship experimental test, and elaborate on them from three aspects, including the flow fields around surface ships and underwater vehicles, as well as ship air flow fields. Finally, the direction for the R&D of PIV technology in ship fields is put forward. In terms of the speed measurement hardware system, PIV algorithms and ship flow field research, several problems requiring further study are highlighted, suggestions on the measurement of ship flow field are given. This study has certain reference and guiding significance for future research on flow field performance optimization.
2021, 16(6): 92-98. doi: 10.19693/j.issn.1673-3185.02051
Abstract:
Objectives  In order to reduce the flow noise on the outer surface of the traditional circular sonar, three sonar shapes (circular, elliptical and square) are proposed, and the hydrodynamic characteristics and flow noise on their outer surfaces are studied.  Methods  Based on the standard \begin{document}$k - \varepsilon$\end{document} turbulence model in Fluent and Lighthill's acoustic analogy method, the flow field and sound field of the outer surfaces of three alternative sonars are analyzed.  Results  Comparing the hydrodynamic results of the three sonar shapes, it is found that the square has the largest lift coefficient amplitude and average drag coefficient, and the ellipse has the smallest. The hydrodynamic diversity is caused by the differences in the boundary layer separation points and wake vortex. Comparing the flow noise results, it is found that the total sound pressure level of the circle is the largest and that of the ellipse is the smallest. The circle and square show the characteristics of a "positive figure eight" dipole sound source, and the maximum sound radiation is perpendicular to the incoming flow direction, while the ellipse shows the characteristics of an "inverted figure eight" dipole sound source, and the maximum sound radiation is horizontal to the incoming flow direction.  Conclusions  This study can provide references for the shape design of sonar, e.g., replacing the traditional circular sonar with the elliptical sonar.
2021, 16(6): 99-108. doi: 10.19693/j.issn.1673-3185.02099
Abstract:
Objectives  The existing combat deduction simulation system mainly implements decision-making based on operational rules and experience knowledge, and it has certain problems such as limited application scenarios, low decision-making efficiency and poor flexibility. In view of the shortcomings of conventional decision-making methods, an intelligent decision-making model based on deep reinforcement learning (DRL) technology is proposed.   Methods  First, the maximum entropy Markov decision process（MDP） of simulation deduction is established, and then the agent training network is constructed on the basis of actor-critic architecture to generate randomization policies that improve the agent's exploration ability. At the same time, the soft policy iterative updating method is used to search for better policies and continuously improve the agent's decision-making level. Finally, the simulation is carried out on the Mozi AI platform to validate the model.   Results  The results show that an agent trained with the improved soft actor-critic (SAC) decision-making algorithm can achieve autonomous decision-making. Compared with the deep deterministic policy gradient (DDPG) algorithm, the probability of winning is increased by 24.53%.   Conclusions  The design scheme of this decision-making model can provide theoretical references for research on intelligent decision-making technology, giving it some reference significance for warfare simulation and deduction.
2021, 16(6): 109-115. doi: 10.19693/j.issn.1673-3185.02096
Abstract:
Launched from a sonobuoy tube and capable of autonomous flight, the autonomous sonobuoy frees the traditional sonobuoy from dependence on costly aerial delivery platforms. It can be installed on various small and medium-sized surface ships and unmanned vehicles to support the operational needs of long-distance and rapid situation awareness regarding underwater battlefields. By introducing the development of in-tube launch and sonobuoy technology, this paper proposes the design method of the autonomous sonobuoy and summarizes the key technologies of autonomous sonobuoy cluster operations, including cluster control, intelligent decision-making, network detection and so on. Furthermore, the combat patterns of autonomous sonobuoy clusters in wide-area cooperative detection, multi-source information support and evaluation assistance of combat effectiveness are analyzed. The results can provide references for the demand analysis, design and application of autonomous sonobuoy clusters.
2021, 16(6): 116-123, 182. doi: 10.19693/j.issn.1673-3185.02018
Abstract:
Objectives   Transmitting extreme low frequency (ELF) signals with a high signal-noise ratio (SNR) to cover Chinese territory and major territorial sea areas can provide a radiating signal service for trial and exploratory research projects such as underground resource sounding, seismic prediction, etc.   Methods   This paper explains the primary principle of horizontal low-profile antennae in radiating ELF signals, introduces ELF transmission system composition, analyzes key technologies such as ELF power amplification, horizontal antennae tuning, detection and control of strong power electromagnetic fields, composite lightning-proof measures, etc., and designs and constructs the first-ever civilian ELF transmission station in the world, through which long-range field tests and trials with a coverage of thousands of kilometers are accomplished.   Results   The test and trial results indicate that ELF signals can cover Chinese territory and major territorial sea area 10-20 dB higher than natural signals, with a sounding depth of up to 10 km.   Conclusions   The ELF transmission station can provide a brand-new open public platform for research on geological exploration and seismic prediction, thereby contributing to the development of primary disciplines in China including low-frequency radioactivity, space physics, geophysics, etc.
2021, 16(6): 124-131. doi: 10.19693/j.issn.1673-3185.02197
Abstract:
Objective   In this paper, to address sparse feature points and unique epipolar constraints, an adaptive depth constraint-based underwater feature matching (ADC-UFM) scheme is proposed.   Methods  By combining a features from accelerated segment test (FAST) operator with scale invariant feature transform (SIFT) descriptors, the matching accuracy can be dramatically improved. By introducing an underwater refractive factor, the matching constraint model (MCM) can be effectively established, thereby contributing to eliminating mismatched points. The adaptive threshold choosing (ATC) module is finely devised to preserve image feature information in changeable underwater environments to an extreme extent.   Results  Comprehensive experiments show that the proposed ADC-UFM scheme can outperform typical matching schemes including SIFT, speeded-up robust features (SURF) and SIFT feature matching based on underwater curve constraint (UCC-SIFT), which not only achieves 85.2% matching accuracy but also meets the real-time requirements.   Conclusion  The results of this study can provide a reliable guarantee for subsequent underwater 3D reconstruction based on the binocular vision system.
2021, 16(6): 132-139. doi: 10.19693/j.issn.1673-3185.02107
Abstract:
Objectives  Aiming at the poor control effect of the traditional adaptive filtering algorithm for multi-frequency excitation in vibration control, and the engineering problems of difficult sensor installation and channel coupling that cause reference signal mismatch, this paper proposes a feedback multi-line spectrum control algorithm.  Methods  First, the error signal is passed through the cascaded adaptive notch filter, and the notch filter parameters are updated according to the adaptive algorithm to estimate multiple signal frequencies. Next, each reference signal is synthesized and phase compensation performed, then another reference signal is obtained through Hilbert transform and the parallel controller is finally entered to complete the amplitude update and realize vibration control.  Results  Through simulation and experimental verification, the results show that the proposed algorithm can accurately estimate frequency information, synthesize reliable reference signals and achieve 20–40 dB energy attenuation for 30, 37, 60 and 110 Hz line spectrums.  Conclusions  This algorithm provides a better solution to the problems of reference signal mismatch and multi-line spectrum vibration in vibration control, and effectively reduces and suppresses low-frequency vibration energy transmission.
2021, 16(6): 140-150. doi: 10.19693/j.issn.1673-3185.02121
Abstract:
2021, 16(6): 151-158. doi: 10.19693/j.issn.1673-3185.02115
Abstract:
Objectives  In order to design a marine internal pressure resistant square cabin which meets the requirements for strength and lightweight design, the neural network surrogate model is combined with heuristic intelligent optimization algorithms and applied to the shape and size optimization of the components of such a cabin.  Methods  The corner chamfer radius, plate thickness and beam model number are selected as design variables for conducting three-dimensional parametric modeling, and sample points are selected according to the optimal Latin hypercube experimental design method. The response values of these sample points are then calculated to build a radial basis functions (RBF) neural network surrogate model. To perform global optimization, the surrogate model is combined with three heuristic optimization algorithms respectively: an adaptive simulated annealing algorithm (ASA), multi-island genetic algorithm (MIGA) and particle swarm optimization (PSO) algorithm.  Results  The results show that the three hybrid optimization methods can all reduce structural weight on the basis of meeting the allowable strength requirements, and the optimal solution sought by the RBF-ASA method in the overall situation has a relatively good weight reduction effect.  Conclusions  This study can provide valuable references for the optimal design of internal pressure-resistant square cabin structures, giving it great significance for overcoming the key technical problems faced by ships using nuclear power plants.
2021, 16(6): 159-165. doi: 10.19693/j.issn.1673-3185.02055
Abstract:
Objectives  The design domain for the topological optimization of ship structures is usually based on two-dimensional shell elements. However, the thickness difference of the design domain affects the setting of volume fraction constraints and the acquisition of stable topological configuration, restricting the practical application of the topology optimization method in the field of hull structure design. Thus, relevant research needs to be carried out to solve this problem.  Methods  This paper takes the transverse web frame in the cargo tank of a very large crude oil carrier (VLCC) as the optimization object, and puts forward a compromise method for setting the volume fraction constraint value of the complex hull structure, as well as a method for determining the minimum stable topological plate thickness of the transverse web frame via element statistics.  Results  Through theoretical analysis and trial calculation, it is found that the volume fraction value and minimum stable topological plate thickness can be obtained more reliably using this method.  Conclusions  The proposed method possesses validity and feasibility, and is able to provide technical support for the optimization design of the transverse web frames of large oil tankers. It can also provide references for the topology optimization of other complex hull structures.
2021, 16(6): 166-175. doi: 10.19693/j.issn.1673-3185.02041
Abstract:
Objectives  The welding distortion of block structures in the fabrication of semi-submersible lifting and disassembly platforms influences their dimensional accuracy and production schedule.  Methods  Thus, taking a B514 block as the research object, the tendency and magnitude of its out-of-plane welding distortion are predicted in advance, and the influence of inverse deformation and welding sequence are examined for fabrication accuracy enhancement. Next, typical welded joints and their welding conditions are summarized, and effective thermal-elastic-plastic finite element (FE) computation is carried out to examine the thermal-mechanical response and evaluate the inherent welding deformation. With inherent deformation as mechanical loading, elastic FE analysis is then employed to predict the dimensional accuracy of the B514 block. Moreover, the influences of inverse deformation application and different welding sequences on dimensional accuracy are also examined.  Results  Based on elastic FE analysis with inherent deformation, the computed out-of-plane welding distortion is found to be in good agreement with the measurement, and its generation mechanism is also clarified.  Conclusions  Effective FE analysis can be employed to predict the inherent deformation and distortion of complex welded structures; moreover, inverse deformation application and an optimized welding sequence can significantly reduce welding distortion.
2021, 16(6): 176-182. doi: 10.19693/j.issn.1673-3185.02134
Abstract:
Objectives  This paper aims to study the fatigue damage problem of large ship prone to springing effect.  Methods  The test result in two generation 400 000 DWT very large ore carrier (VLOC) are compared based on ship model test method, and the relation of sea states and fatigue including hull girder springing are analyzed. According to the recursion method, a classical sea state is selected from a series of model test sea states.  Results  The results show that the fatigue damage magnification effect has a trend relationship with significant wave height and encounter period. The concept of "dominant sea state" is proposed based on the recurrence method, and the expression of fatigue damage magnification factor is improved. The springing effect varies significantly according to different sea states, and the fatigue damage magnification effect of springing increases very rapidly as significant wave height and encounter period decrease.  Conclusions  Replacing multiple model test loading conditions with dominant sea state could allow relatively accurate fatigue damage magnification factor to be ascertained rapidly. As such, this paper can provide references during the initial design period.
2021, 16(6): 183-190. doi: 10.19693/j.issn.1673-3185.02158
Abstract:
Objectives   This paper proposes a method for effectively extracting fault features and identifying fault patterns from the early impact vibration signals of the rolling bearings of complex equipment which is non-stationary, nonlinear and has strong background noise.   Methods   First, the fault features of the original vibration signals are extracted via fast spectral correlation analysis and quantified via multi-scale permutation entropy (FSC-MPE). The fault feature data is then input into a BP neural network for fault diagnosis model training and testing. Finally, fault identification research is carried out on the rolling bearing fault simulation experimental data under variable speed and the public bearing fault test dataset of Case Western Reserve University.   Results   The results show that the proposed method has high identification accuracy for different types of faults, reaching more than 97%.   Conclusions   The feasibility and superiority of the proposed rolling bearing fault diagnosis method based on FSC-MPE and BP neural network are verified, and it can provide technical support for rolling bearing health evaluation.
2021, 16(6): 191-200. doi: 10.19693/j.issn.1673-3185.02176
Abstract:
Objective  A multi-input multi-output (MIMO) robust controller is proposed to realize the trajectory tracking and yaw control of a four-DOF Ciscrea autonomous underwater vehicle (AUV) model.   Methods  The parameter uncertainty of the inertia matrix and quadratic damping action is formulated as an uncertain structure via the perturbation method, and the general system is derived by linear fraction transformation (LFT). The H-infinity synthesis method is applied to solve the MIMO robust controller for the AUV general system, and the structure singular value analysis method is used to compute the robust stability margin. To validate the robust yaw controller, AUV's three-dimensional trajectory tracking and yaw control scenarios are simulated using MATLAB. The tracking performance is compared between interference and non-interference control conditions.   Results  The upper and lower bounds of the structure singular value are obtained for the stable robust controller.The elimination of perturbation on AUV output shows effective anti-jamming performance.   Conclusions  The proposed control method is available for solving the parameter uncertainty and nonlinearity issues of AUV models, which can provides a specific application for addressing AUV motion and attitude control problems in real ocean environments.
2021, 16(6): 201-208, 215. doi: 10.19693/j.issn.1673-3185.02265
Abstract:
Objectives  This paper studies the dynamic and tribological characteristics of the main journal bearing, and the coupling relationship between them.   Methods  First, a dynamic model of the coupling tribological properties of a bearing wear test bed is established using the substructure method. The friction power consumption and orbital paths of the bearing are then calculated and compared with the measured results to verify the accuracy of the model. Finally, based on the model, the dynamic and tribological coupling analyses of the bearing are carried out.   Results  The results show that the four support bearings of the test bed are in a state of hydrodynamic lubrication, while the tested bearing is in a state of mixed lubrication at the top dead center. With the increase in radial clearance, the minimum oil film thickness of the tested bearing increases first and then decreases, and the most optimal lubrication state can be obtained when the tested bearing clearance is about 20 μm. Compared with the oil film model of a nonlinear spring, the elastohydrodynamic model can provide bearing load and friction power consumption results with higher accuracy.   Conclusions   The results of this study can provide theoretical guidance for the lubrication performance design and high-precision modeling of bearing pairs.
2021, 16(6): 209-215. doi: 10.19693/j.issn.1673-3185.02089
Abstract:
Objectives  Based on the operation requirements of the steam dump system of marine nuclear power plant, the dynamic characteristics and influencing factors of steam dump conditions are analyzed.   Methods  According to the functional characteristics of the steam dump system, mathematical and physical models of the matching operations of the steam generator, steam system, bypass system and main steam turbine are established. In order to verify the accuracy and universality of the simulation model, an analysis is made of the dynamic operation characteristics and coupling matching characteristics of the primary and secondary circuits under extreme working conditions, such as the quick shut-off operation of the main stop valve and load rejection of the steam turbine.   Results  The simulation results show that when operating under steam dump conditions, the appropriate steam dump valve, main stop valve and their matching action time should be selected according to the power self-regulation capacity of the marine reactor, and safety problems such as the overpressure of the steam header and overspeed of the steam turbine unit should be comprehensively considered. At the same time, the steam demands of other equipments on the ship should be met.   Conclusions  The results of this study can provide references for the safety control of steam dump conditions and the optimal design of the steam dump systems of marine nuclear power plants.
2021, 16(6): 216-224, 230. doi: 10.19693/j.issn.1673-3185.02173
Abstract:
Objective  To prevent the marine biofouling adhesion and corrosion of ship hulls and pipes, a system is designed which produces chlorine through seawater electrolysis using an ion-exchange membrane electrolytic bath. Thereby, the law and efficency of the electrolytic bath under different conditions is studied.   Methods  First, the effects of brine temperature, brine concentration, current density and time of brine resided in the bath on the efficiency of the electrolysis process are investigated. Minitab software is then used to optimize the parameters of the electrolysis process using the specific energy consumption rate as the evaluation index. Finally, the seawater pretreatment process and electrolysis process are verified on site under real sea conditions.   Results  The current density and residence time of the electrolysis parameters were reasonably controlled at 3 000 A/m2 and 46 s respectively, in this case, the current efficiency was higher than 80%, the cell voltage was lower than 6 V, and the surfaces of the anodes, cathodes and ion-exchange membranes after electrolysis were clean.   Conclusion  The practical tests show that this system can produce chlorine for anti-fouling.
2021, 16(6): 225-230. doi: 10.19693/j.issn.1673-3185.02179
Abstract:
Objectives   In order to improve the efficiency of sodium persulfate (Na2S2O8) wet oxidation denitrification and its utilization rate, wet oxidation denitrification experiments using hydrodynamic cavitation to enhance Na2S2O8 are carried out.   Methods   This paper explores the feasibility of using hydraulic cavitation to enhance Na2S2O8 denitrification, and studies Na2S2O8 solution temperature and concentration, chloride ions (Cl) and other factors on denitrification effects and related mechanisms.   Results   The experimental results show that the special environment created by hydrodynamic cavitation is beneficial for activating Na2S2O8 and improving its denitrification efficiency, but the overall denitrification effect is insufficient. Under the experimental conditions, when the temperature increases from 30 °C to 80 °C, the nitrigen oxide (NO) removal rate increases from 7% to 43%, showing that increasing the concentration of Na2S2O8 is beneficial for increasing the NO removal rate. Under high-temperature conditions, increasing the concentration of Na2S2O8 has a more obvious promotion effect on improving the overall denitrification rate. The presence of Clcan greatly increase the NO removal rate. When the ratio of Cl concentration to Na2S2O8 concentration reaches 0.15∶0.1, the NO removal rate reaches 94%.   Conclusions   Hydrodynamic cavitation promotes the reaction rate of Na2S2O8 to remove NO. Cl produces more oxidizing substances in the special reaction environment created by hydrodynamic cavitation, thereby improving the denitrification effect.