Feature Extraction Methods for Underwater Acoustic Target Recognition of Divers.

The extraction of typical features of underwater target signals and excellent recognition algorithms are the keys to achieving underwater acoustic target recognition of divers. This paper proposes a feature extraction method for diver signals: frequency-domain multi-sub-band energy (FMSE), aiming to achieve accurate recognition of diver underwater acoustic targets by passive sonar. The impact of the presence or absence of targets, different numbers of targets, different signal-to-noise ratios, and different detection distances on this method was studied based on experimental data under different conditions, such as water pools and lakes. It was found that the FMSE method has the best robustness and performance compared with two other signal feature extraction methods: mel frequency cepstral coefficient filtering and gammatone frequency cepstral coefficient filtering. Combined with the commonly used recognition algorithm of support vector machines, the FMSE method can achieve a comprehensive recognition accuracy of over 94% for frogman underwater acoustic targets. This indicates that the FMSE method is suitable for underwater acoustic recognition of diver targets.

Stiffness analysis and structural optimization design of an air spring for ships.

An air spring (AS) for ships must have the structural strength of its bellows enhanced considerably to ensure its reliability under high internal pressure and strong impact. In this case, the stiffness of the bellows gradually dominates the overall stiffness of the AS. Nevertheless, the parameterization calculation of stiffness for an AS mainly focuses on its pneumatic stiffness. The bellows stiffness is normally analyzed by virtue of equivalent simplification or numeric simulation. There is not an effective parameterization calculation model for the stiffness of the bellows, making it difficult to achieve the structural optimization design of the bellows. In this paper, the shell theory was borrowed to build a mechanical model for the bellows. Subsequently, the state vector of the bellows was solved by precision integration and boundary condition. Iteration was conducted to identify the complex coupling relationship between the vector of the bellows and other parameters. On this basis, the parameterization calculation method was introduced for the stiffness of the bellows to obtain the vertical and horizontal stiffness of the AS. After that, a dual-membrane low-stiffness structure was designed to analyze the dominating factors affecting the strength and stiffness of the AS, which highlighted the way to the low-stiffness optimization design of high-strength ASs. In the end, three prototypes and one optimized prototype were tested to verify the correctness of the parameterization design model for stiffness as well as the effectiveness of the structural optimization design.

Mechanical characteristics analysis of high dimensional vibration isolation systems based on high-static-low-dynamic stiffness technology.

Large floating raft vibration isolation systems (FRVISs) based on high-static-low-dynamic stiffness (HSLDS) technology offer excellent low frequency vibration isolation performance with broad application prospects. However, the design process for these complex high-dimensional coupled nonlinear systems remains poorly developed, particularly when applied for ocean-going vessels that experience rolling and pitching motions. The present work addresses this issue by establishing a six-degree-of-freedom HSLDS vibration isolation model for FRVISs composed of eight isolators, and the model is applied to fully analyze the swing stability and multidimensional vibration isolation performance of these systems. The influence of nonlinearity on the mechanical properties of the vibration isolators is analyzed more clearly by assuming that each vibration isolator realizes nonlinear HSLDS characteristics in the z direction and linear characteristics in the x and y directions. The results demonstrate that the swing displacement responses of the system are greatly reduced under weak nonlinearity, which reflects the high static stiffness and high static stability characteristics of an HSLDS system. The multidimensional vibration isolation performance of the system is evaluated according to the impacts of nonlinearity, the installation height Hz of the isolators, and the relative position Dr of the two middle isolators. The results of analysis demonstrate that applying a value of Hz = 0 produces the best vibration isolation performance overall under strong nonlinearity by avoiding unnecessary secondary peaks in the force transmission rate under harmonic mechanical excitation and ensuring a maximum high-frequency vibration isolation effect. However, applying a weak nonlinearity is better than a strong nonlinearity if Hz is not zero. In contrast, Dr has little effect on the vibration isolation effect of the raft in the x, y, and z directions. Therefore, an equidistant installation with Dr = 0.5 would be considered ideal from the standpoint of installation stability.

Magnetic field model of radially magnetized cylindrical permanent magnets in a self-biased magnetic pendulum array.

Self-Biased Magnetic Pendulum Array (SBMPA) is an efficient and portable transmitter in ultralow frequency (ULF). The resonance frequency of SBMPA is affected by the magnetic field of the radially magnetized cylindrical permanent magnets. In order to calculate the resonance frequency, the magnetic field model of a single radially magnetized cylindrical permanent magnet is derived based on the concept of magnetic charge. Then, the influence of the demagnetizing field and external magnetic field on the magnetization of permanent magnets is analyzed, and the magnetic field model of SBMPA is established. The results of the magnetic field model are verified through simulation. The average deviation of magnetic field intensity is determined as 0.021%; thus, the magnetic field model and simulation have consistent results. Finally, the influence of the size and distance of permanent magnets on the resonance frequency is analyzed, which could provide theoretical guidance for the dynamic analysis of SBMPA.

Research on the Friction Noise Generation Mechanism and Suppression Method of Submarine Rubber-Based Propeller Bearings-A Review.

This article introduces the main mechanisms of friction noise generated by submarine rubber-based propeller bearings and analyzes their respective scope of application and limitations. Then, the research on suppressing friction noise through the optimization of the structure and improvement of materials of rubber-based propeller bearings is discussed. Finally, the article summarizes a promising research direction aimed at eliminating friction noise in submarine rubber-based propeller bearings. By improving the structure and materials, the friction noise of propeller bearings can be effectively suppressed, thereby improving the deterrence and stealth performance of submarines.

Bellows stiffness characteristics of cord-reinforced air spring with winding formation under preload conditions.

The bellows structure in an air spring can be constantly reinforced to cope with the complicated work environment, but it exerts a stronger and stronger effect on the stiffness characteristics of the air spring. However, there is not any effective way for the parameterized solution of the bellows stiffness of the air spring. With the precise transfer matrix method, the bellows stiffness characteristics of a cord-reinforced air spring with winding formation under preload conditions were analyzed in this paper. The thin-shell theory was used to solve the bellows pre-stress of the air spring under preload conditions. The pre-stress was introduced into the equilibrium equation for the bellows. Based on the geometrical and physical equations for the bellows with the complex cord winding characteristics, the precise integration method was borrowed to construct a transfer matrix for the bellows of the air spring under preload conditions. The state vector of the bellows in the air spring was solved through boundary conditions. The iteration method was adopted to develop the expression for the bellows stiffness characteristics, and combined with the theoretical model of pneumatic stiffness to solve the stiffness characteristics of the air spring. The comparison with the prototype test results verified the validity and correctness of the theoretical model. On this basis, we explored the influence of preload conditions, geometrical structure, and material characteristics on the stiffness characteristics of the air spring. The research findings will provide significant guidance for the structural design and material selection of cord-reinforced air springs with winding formation.

Study on the Actuation Properties of Polyurethane Fiber Membranes Filled with PEG-SWNTs Dielectric Microcapsules.

Polyurethane dielectric elastomer (PUDE), a typical representative of emerging intelligent materials, has advantages, such as good elasticity and flexibility, fast response speed, high electromechanical conversion efficiency, and strong environmental tolerance. It has promising applications in underwater bionic actuators, but its electromechanical properties should be improved further. In this context, the design of polyethylene glycol (PEG) single-walled carbon nanotube (SWNTs) dielectric microcapsules was adopted to balance the problem of contradictions, which conventional dielectric modification methods face between comprehensive properties (e.g., dielectric properties and modulus). Moreover, the dielectric microcapsule was evenly filled into the polyurethane fiber by coaxial spinning technology to enhance the actuation performance and instability of the electrical breakdown threshold of conventional polyurethane dielectric modification. It was revealed that the dielectric microcapsules were oriented in the polyurethane fiber, and the actuation performance of the composite fiber membrane was significantly better than that of the polyurethane fiber membrane filled with SWNTs, thus confirming that the filling design of the dielectric microcapsules in polyurethane fiber could have certain technical advantages. On that basis, this study provides a novel idea for the dielectric modification of polyurethane.

The Effect of a Flexible Electrode on the Electro Deformability of an Actuating Unit of a MDI-Polyurethane Composite Fiber Membrane Filled with BaTiO3.

The electro deformability of an actuating unit of a polyurethane dielectric elastomer (PUDE) is affected by many factors. The agglomeration of dielectric fillers faced by the traditional dielectric modification methods will lead to the instability of the actuation performance of dielectric composites. In addition, the electro deformability (ability of deformation after voltage loading) is great affected by the selection of flexible electrodes and packaging technology. Based on the research findings, Diphenylmethane-4,4'-diisocyanat (MDI)-polyurethane dielectric composite fiber membrane filled with barium titanate (BaTiO3) is prepared using coaxial spinning, and this study then analyzes the effects of the types of flexible electrodes and coating methods on the electro deformability of the actuating unit of the dielectric composite fiber membrane. It is found that the electro deformability of the actuating unit coated with the single-walled carbon nanotube (SWNT) flexible electrode is better than that of the perfluoropolyether conductive grease (PCG) or the traditional conductive carbon grease (CCG) electrode in various degrees. When the loading voltage is 20 kV, the electro deformability of the actuating unit coated with SWNT flexible electrode exceeds the latter two electrodes by 13.8%; when the SWNT flexible electrode is encapsulated by physical surface implantation (PSI), the electric deformation of the actuating unit is higher than that of the solvent suspension dispersion (SSD).

Preparation and Dielectric Sensitivity of Polyurethane Composite Fiber Membrane Filled with BaTiO3.

Polyurethane dielectric elastomer (PUDE) is considered a potential underwater flexible actuator material due to its excellent designability and environmental tolerance at the molecular level. Currently, the application of the polyurethane elastomer as an actuating material is constrained by such problems as the conflict between various properties such as dielectric properties and modulus and the low level of dielectric sensitivity. This is a common challenge facing polyurethane dielectric research related to the uneven distribution of dielectric fillers in the matrix. Besides, another challenge for the academic circles is the easy agglomeration of micro and nanofillers. Given the above-mentioned background of the application and technical problems, the coaxial electrospinning technology is proposed in this paper. The polyurethane fiber network is constructed with the preferred hydrolysis resistant polyether-Diphenylmethane diisocyanate (MDI) thermoplastic polyurethane elastomer as the matrix material. Dispersed by ultrasound, the micro nano dielectric filler is integrated into polyurethane fiber through the coaxial dual-channel design. Additionally, directional constraint molding is conducted to improve the agglomeration of small-scale particles induced by the loss of mechanical energy in traditional blending. After characterization, the distribution of BaTiO3 particles in the fiber bundle is relatively uniform. Compared to the polyurethane dielectric composites prepared by traditional blending (BaTiO3-Dielectric Elastomer, BaTiO3-DE), the dielectric sensitivity factor of the polyurethane composite fiber membrane (BaTiO3-Dielectric Elastomer Membrane, BaTiO3-DEM) is enhanced by over 25%; the electrostrictive strain of BaTiO3-DEM is boosted by least 10%.

The Synergistic Effect of WS2 and SWNTs on Tribological Performance of Polyether MDI Polyurethane Elastomer under Dry and Wet Friction Conditions.

To adapt to the complex application of polyurethane bearings, it is feasible to improve the tribological performance of single polyurethane-based friction materials through the synergistic effect produced by multi-component-lubricating fillers. In this context, rather than using tungsten disulfide (WS2), which has demonstrated excellent self-lubricating performance as a lubricating oil additive, this paper proposes that WS2 and single-walled carbon nanotubes (SWNTs) can be designed for addition into a polyether 4,4'-diphenylmethane diisocyanate (MDI) polyurethane matrix as self-lubricating fillers so as to explore the synergistic effect of micro- and nano-lubricating fillers on the tribological performance of polyurethane matrix materials. Through a series of characterizations and tests, it was found that the dispersion of two-component-lubricating additives in a polyurethane matrix is improved when the ratio of WS2 to SWNTs is roughly 2:1. In this case, the tribological performance of polyurethane matrix composites is more satisfactory than at other ratios. In addition, compared with the blank sample, the tribological performance of the synergistically modified polyurethane composites under dry friction is more significantly improved with the increase in contact load, while there is no significant improvement under water lubrication. Aside from contributing to the idea of exploring the synergistic effect of WS2 and other micro or nanofillers, this method also opens up the possibility of practically applying WS2 in the field of friction.

Stability of filament-wound hyperbolic flexible pipes under internal pressure based on non-geodesic winding.

Filament-wound flexible pipes are widely used to transport fluid in pipeline systems, proved extremely useful in marine engineering. The hyperbolic flexible pipes have good vibration suppression performance, but they are easily deformed under internal pressure. This paper focuses on the stability of hyperbolic flexible pipes based on the composite Reissner shell theory and the transfer-matrix method. The nonlinear stretch of the reinforced filament and the fiber bridge effect are considered in the model. The calculation results show that a large winding angle reduces the deformation and the meridional stress. The available initial winding angle is limited by the geometry and the slippage coefficient of flexible pipe. The reinforced filament of high tensile modulus will reduce the deformation of the pipe. Compared with the geodesic winding trajectory, non-geodesic winding trajectories improves the stability of the pipe. The theoretical result is verified by the finite element analysis. The investigation method and results present in this paper will guide the design and optimization of more novel flexible pipes in the future.