Search

Periodic isolation system is effectively applied in broadband vibration control. To further enhance the broadband vibration attenuation effect, the paper proposes a two-stage periodic isolation system for an underwater glider. The analytical model of the one-dimensional two-stage periodic isolation system is developed through the multi-degree of freedom spring mass model method. For illustrating the superiority of the proposed two-stage periodic isolator, the force transmission ratio and the wave propagation constant of the SDOF isolator, the single-stage periodic isolator, and the two-stage periodic isolator are calculated and compared. In order to obtain the dynamic parameter influences on the vibration isolation performances as the design guidelines of the two-stage periodic isolator, the parametrical study is carried out based on the analytical model. Furthermore, a two-stage periodic isolator is designed for an underwater glider. The application effect of the two-stage periodic isolator is investigated through analytical modeling and finite element method, comparing to the single-stage periodic isolator. The research results from the analytical models show the proposed two-stage periodic isolator could strength the broadband vibration suppression. The parametrical study results demonstrate the vibration attenuation bandgap and attenuation amount are greatly influenced by the designed dynamic parameters, such as the mass unit and the spring unit of the periodic isolator, the intermediate mass of the two-stage isolator, and the number of the periodic cells. In application study of an underwater glider, the finite element results verify that the two-stage periodic isolator has more vibration attenuation effect than the single-stage periodic isolator. The vibration isolation assessment according to the proposed analytical model gives good predictive performance before the finite element model verification. [ABSTRACT FROM AUTHOR]

The manta ray, exemplifying an agile swimming mode identified as the median and paired fin (MPF) mode, inspired the development of underwater robots. Robotic manta typically comprises a central rigid body and flexible pectoral fins. Flexible fins provide excellent maneuverability. However, due to the complexity of material mechanics and hydrodynamics, its dynamics are rarely studied, which is crucial for the advanced control of robotic manta (such as trajectory tracking, obstacle avoidance, etc.). In this paper, we develop a multibody dynamic model for our novel manta robot by introducing a pseudo-rigid body (PRB) model to consider passive deformation in the spanwise direction of the pectoral fins while avoiding intricate modeling. In addressing the rigid-flexible coupling dynamics between flexible fins and the actuation mechanism, we employ a sequential coupling technique commonly used in fluid-structure interaction (FSI) problems. Numerical examples are provided to validate the MPF mode and demonstrate the effectiveness of the dynamic model. We show that our model performs well in the rigid-flexible coupling analysis of the manta robot. In addition to the straight-swimming scenario, we elucidate the viability of tailoring turning gaits through systematic variations in input parameters. Moreover, compared with finite element and CFD methods, the PRB method has high computational efficiency in rigid-flexible coupling problems. Its potential for real-time computation opens up possibilities for future model-based control. [ABSTRACT FROM AUTHOR]

This study aims to reveal the buckling behavior of filament-wound composite cylindrical shells subjected to external pressure. The boundary conditions of the cylindrical shells were one end fixed and the other free. The carbon fiber stacking sequences were [±90]2/([±20]/[±90]/[±40]/[±90]/[±60]/[±90])2/[±90]. Finite element software ANSYS 16.2 was used for the numerical simulation to predict the critical buckling pressure and buckling behavior of composite cylindrical shell. External hydrostatic pressure tests were conducted, where the buckling behavior and strain response were observed. Numerical simulation accurately predicted the critical buckling pressure of carbon fiber/epoxy filament composite cylindrical shells under external pressure with 3.5% deviation from the experimental results. The buckling modes simulated by the finite element method agreed well with the deformed shape observed in the experiment, which was characterized by the uniform distribution of the three hoop waves. Comparing the axial compressive strain and hoop compressive strain of the composite shell, it was found that the circumferential stiffness of the shell was weaker than the axial stiffness. In addition, a comparative study of the strains of the fixed-end and free-end metal control sleeves was carried out. The results show that the boundary conditions have a significant influence on the strain response of control sleeves. [ABSTRACT FROM AUTHOR]

In this paper, water entry process of air launched AUV is investigated by employing fully coupled finite element method and arbitrary Lagrange–Euler formulation (FEM-ALE) and using penalty coupling technique. Numerical model is established to describe the hydrodynamic characteristics and flow patterns of a high-speed water entry AUV. The effectiveness and accuracy of the numerical simulation are verified quantitatively by the experiments of the earlier study. Selection of suitable advection method and mesh convergence study is carried out during experimental validation process. It is found that appropriate mesh size of impact domain is crucial for numerical simulations and second-order Van Leer advection method is more appropriate for high speed water entry problems. Subsequently, the arbitrary Lagrange–Euler (ALE) algorithm is used to describe the variation laws of the impact load characteristics with water entry velocities, water entry angles and different AUV masses. Dimensionless impact coefficient of AUV at different velocities calculated using ALE method is compared with SPH results. This reveals that ALE method can also simulate the water entry process accurately with less computational cost. This research work can provide beneficial reference information for structure design of AUV and for selection of the water entry parameters. [ABSTRACT FROM AUTHOR]

This paper is devoted to solving the stacking sequence optimization problem of symmetrical laminated composite cylindrical shells subjected to hydrostatic pressure. First, a conventional genetic algorithm (GA) coupled with a finite element analysis optimization method is developed to search for the best laminations with the maximum buckling pressure. These optimal laminations share similar extensional stiffness coefficient ratios A 11 / A 22 and bending stiffness coefficient ratios D 11 / D 22 because the two ratios of the optimal lamination fluctuate slightly around a specific value. Based on this phenomenon, a stiffness coefficient-based method (SCBM) is then proposed. The method is integrated with the GA and the stiffness coefficient calculation to identify the lamination whose stiffness coefficient ratios (A 11 / A 22 and D 11 / D 22) are closest to those obtained previously. The effectiveness of the SCBM is validated by comparison with the optimal results. The proposed method is then extended to more complex symmetrical laminations. Finite element analysis is also coupled with the GA as a control group. Comparisons reveal that the two methods lead to similar characteristic lamination patterns and maximum buckling pressures, which suggests that the SCBM works well for complex laminations. Moreover, the SCBM is found to be significantly more efficient because it only needs to calculate the stiffness coefficients rather than analyse the entire structure during the optimization. • The stiffness coefficient ratios of the optimal laminations are found to fluctuate slightly around a specific value. • The SCBM is proposed to find a lamination with stiffness coefficient ratios closest to this specific value. • The proposed SCBM proved to be more efficient than the most commonly used GA-FEM. [ABSTRACT FROM AUTHOR]

Abstract Air-launched autonomous underwater vehicles (AUVs) are subjected to huge impact loads in the early stage of water entry, which may cause structural damage or failure of electronic components, especially in the case of high speed water entry. Therefore, it is very imperative to carry out experimental and numerical research on the impact loads of air-launched AUVs. An experimental study of the water entry of air-launched AUVs with different launch velocities and angles was conducted using high-speed photography and sensing technology. The axial and radial accelerations of AUVs under different working conditions at the early stage of water entry were obtained. Furthermore, a coupled finite element technique and the smooth particle hydrodynamics method (FEM-SPH) is employed to model the water entry process of air-launched AUVs. Numerical simulation results such as the peak of impact acceleration were compared with the presented experiment results. The good agreement between the experimental results and the numerical simulation results revealed the capability and accuracy of the numerical algorithm in solving the problem of AUV water entry. Highlights • The water entry problem of full-scale air-launched AUV are studied experimentally. • The capability and accuracy of the FE-SPH method for simulating the water entry problem of AUV are validated. • The effects of initial water entry conditions on the impact acceleration are discussed. [ABSTRACT FROM AUTHOR]

Permanent magnet couplings (PMCs) can convert the dynamic seal of transmission shaft into a static seal, which will significantly improve the transmission efficiency and reliability. Therefore, the radial Halbach PMC in this paper is suitable as the transmission mechanism of deep sea robots. A two-segment Halbach array is adopted in the radial PMC, and the segment arc coefficient can be adjustable. This paper presents the general analytical solutions of the distinctive Halbach PMCs based on scalar magnetic potential and Maxwell stress tensor. The analytical solutions of magnetic field are in good agreement with 2-D finite element analysis (FEA) results. In addition, an initial prototype of the radial Halbach PMC has been fabricated, and the analytical solutions of magnetic torque are compared with 3-D FEA and experiment results. This paper also establishes an optimization procedure for PMCs based on the combination of 3-D FEA, Back Propagation Neural Network (BPNN), and Genetic Algorithm (GA). 3-D FEA is performed to calculate the pull-out torque of the samples from Latin hypercube sampling, then BPNN is used to describe the relationship between the optimization variables and pull-out torque. Finally, GA is applied to solve the optimization problem, and the optimized scheme is proved to be more reasonable with the FEA method. [ABSTRACT FROM AUTHOR]

Vibrations from the power system can significantly affect the working performances (ocean observation) of autonomous underwater gliders (AUGs). In order to reduce the vibration transmission from vibration sources to the precision instruments in AUGs, single- and two-stage vibration isolator rings are designed in this paper. The dynamic models of the single- and two-stage vibration isolation of the AUG are presented. The force transmission ratio of the AUG is calculated in MATLAB code. The influences of the isolator and the structure stiffness are analyzed. The dynamic stiffness of the designed isolators, as an important design parameter, is calculated using the finite element method. The influence of the designed parameter on the dynamic stiffness of the rubber ring isolator is discussed. The coupled vibro-acoustic finite element method is used to analyze the vibration and acoustic response of an AUG with the single- and two-stage vibration isolators. The insertion loss is calculated in order to assess the vibration isolation performance of the single- and two-stage vibration isolators. The results from the dynamic models and the finite element models both show that the vibration isolation performance of the two-stage vibration isolator ring performs better than that of the single-stage vibration isolator ring. [ABSTRACT FROM AUTHOR]

In this study, an analytical solution for the buckling of a composite cylindrical shell subjected to hydrostatic pressure is proposed. The boundary conditions of the composite cylindrical shell are cantilever-like, with one end fixed and the other end connected to a rigid disk. The differential equations are solved using the Galerkin method. The axial displacement of the shell is approximated by the first mode shape of the transverse vibration of the clamped sliding beam. The circumferential displacement and deflection are approximated by the first derivation of the beam function. Based on this solution, an analytical formula enabling prediction of the critical buckling pressure and buckling mode of composite orthotropic cylindrical shells is derived. A finite element analysis and external hydrostatic pressure test are conducted to verify the proposed approach. The efficiency and accuracy of the analytical solution in predicting the critical buckling pressure and buckling mode are demonstrated. [ABSTRACT FROM AUTHOR]

Upheaval buckling behaviors of imperfect subsea PIP (pipe-in-pipe) pipelines on nonlinear foundation are studied using finite element method. A parametric study is performed and the effects of the parameters on the critical force of upheaval buckling are discussed, including initial imperfection amplitude to wavelength ratio (AWR), the characteristic parameter of initial imperfection shape, stiffness ratio, clearance between outer and inner pipes, and soil condition as well as vertical pipe-soil interaction. A simplified empirical formula considering the above factors is presented to determine the critical force of upheaval buckling for subsea PIP pipelines based on dimensional analysis and the results of the parametric study. The results of case studies show that the proposed empirical formula has good accuracy. [ABSTRACT FROM AUTHOR]

The Autonomous Underwater Vehicle (AUV) will suffer severe impact load during the water entry, and its shell structure will undergo elastoplastic deformation, even breakage or damage, which seriously threatens the safety and reliability of the AUV. Therefore, it is necessary to carry out research on the buffering technology of the impact load of the vehicle into the water to ensure that the AUV can enter the water safely and reliably. In this paper, a mitigator is designed to reduce the impact load of the AUV entering the water. Numerical simulations based on the explicit finite element method with an Arbitrary-Lagrangian Eulerian (ALE) solver are carried out to study the mitigation characteristic of the designed mitigator. The accuracy of the model was verified by the experimental results of the water-entry of the small sphere, and the mesh independence was verified to determine the mesh size. Then, the effects of initial conditions such as buffer length, buffer material density, entry velocity and entry angle on the cushioning effect of the buffer head cap were studied. When the vertical water entry velocity is 50 m/s, the impact load reduction ratio is 42.2%; while the water entry angle is 60° and the water entry velocity is 50 m/s, the axial load reduction ratio is 28.8%, and the normal load reduction ratio is 27.8%. These conclusions will be useful for the designing of the crashworthy structural and the analysis of underwater weapon impact. • A water-entry impact buffer designing method has been demonstrated. • The ALE numerical model for mitigator characteristic analysis was built. • The buffer and load reduction performance with different conditions are presented. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic model for the propulsion shaft system of an unmanned underwater vehicle. The model considers the effects of the flexible coupling and motor isolators. The motor bearings, shaft support bearings, thrust bearing and propeller are considered. A multi-degree of freedom isolation model for the motor stator is established. The coupling effects of shaft, motor bearings, shaft support bearings and motor excitation are considered. A finite element mode analysis and forced vibration experiment is conducted to provide the verification. The influences of the shaft speed, flexible coupling stiffness, motor isolation stiffness and installation position on the system vibrations are studied by using the numerical model. This study could provide some guidance for the dynamic analysis and vibration optimization of the propulsion shaft systems of UUVs. • A dynamic model of motor considering the isolation system and rotor-stator relation is presented. • A dynamic model of propulsion shaft system considering the motor, coupling and bearing is presented. • Effects of the isolator stiffness and position on the dynamic responses are studied. [ABSTRACT FROM AUTHOR]

This paper deals with the delamination damage and the effect of buckling behavior on delamination propagation of the composite cylindrical shell subjected to hydrostatic pressure. The model of the composite shell with initial delamination and geometric imperfection is elaborated numerically by using the finite element method. The virtual crack closure technique is employed to calculate the strain energy release rate, and the linear fracture criterion is used to determine the onset and growth of the delamination. The buckling deformation and delamination propagation are monitored by performing the nonlinear buckling analysis. In addition, a parametric study is carried out to investigate the influence of the initial delamination shape, area, depth, and ply orientation on ultimate pressure and delamination propagation path. The results show that local or global buckling of the shell promotes the propagation of delamination, and the ultimate buckling pressure of the shell is more sensitive to the axial initial delamination length. The initial delamination shape, depth, and ply orientation have different influences on the ultimate pressure of the shell and the delamination propagation path, and the corresponding explanations are given in this paper. • The delamination propagation path of the composite shell subjected to hydrostatic pressure is presented. • A transition zone is introduced in the model to improve the convergence of the entire model. • It is found that the interlaminar damage tends to propagate axially rather than circumferentially. • The effects of different initial delamination types on the buckling and delamination propagation of shells are explained in detail. [ABSTRACT FROM AUTHOR]

Optimization of a composite cylindrical pressure hull subjected to hydrostatic pressure with trapezoidal stiffeners is considered in this paper. The composite cylindrical shell is fabricated with carbon fiber reinforced epoxy composite while the stiffener is made of aluminum alloy. First of all, the analytical buckling model for composite cylinders with stiffeners subjected to hydrostatic pressure is derived. Subsequently, the finite element method is used to verify the accuracy of the analytical solution through some examples. After verification, the analytical solution is coupled with the genetic algorithm and then used to optimize the cross-sectional shape of the stiffeners of the composite pressure hull to obtain the maximum buckling pressure. During the optimization process, the relationship between the buckling pressure and each geometrical parameter of the stiffener has been analyzed. The inertia moment of the stiffener section is found to have a good linear relationship with the buckling pressure. Therefore, the inertia moment is used as an objective function to optimize the cross-sectional shape of the stiffeners of the composite pressure hull, and the feasibility of this method has been proved by comparing the optimization results. With high efficiency, this method can be applied to the optimal design of the stiffener shape of a composite pressure shell. • Buckling of composite cylindrical shells with trapezoidal stiffeners. • The optimal cross-sectional shape of stiffeners has been obtained. • The inertia moment of stiffeners has a linear relationship with buckling pressure. • Using the inertia moment as the objective function can obtain expected results. [ABSTRACT FROM AUTHOR]

This paper is devoted to stress control of cylinders during water entry based on the characteristics of bi-material interfaces. Firstly, the influence factors on dynamic stress are studied from the view of elastic mechanics and wave mechanics. It is found that the dynamic stress depends on the material parameters, material layout, natural frequency and deformation. On this basis, a sandwich scheme is proposed to reduce the stress level of the cylinder entering water. Then a sandwich cylinder, two bi-layer cylinders and two monolithic cylinders during water entry are investigated by coupling of smoothed particle hydrodynamics (SPH) and finite element method (FEM). It is found that when the material parameters are given, the deformation and natural frequency of the sandwich cylinder are moderate, but its stress level is significantly lower than that of other cylinders under the same initial conditions, attributing to its unique material layout. Accordingly, the limit velocity of the sandwich cylinder colliding with water is much higher than that of other cylinders. Therefore, the stress control scheme is expected to reduce the possibility of strength failure of cylindrical structure during water entry. • Sandwich scheme is advised to reduce stress levels of cylinders entering water. • The limit velocity of cylinder colliding with water is significant improved. • The stress control function is still effective during high speed water entry. [ABSTRACT FROM AUTHOR]

• Influence factors of dynamic stress are investigated analytically. • Tungsten carbide may increase the impact strength of aluminum structures. • Polyamides may reduce the impact strength of the aluminum structure. This paper focuses on dynamic stress control of bi-material structure under the action of sawtooth shock pulse. The factors that are likely to influence the dynamic stress are studied based on the analytical method. Then two bi-material models and a homogenous material model are investigated through the finite element method. The supplement of tungsten carbide layer could reduce the stress of aluminum alloy significantly are proposed here, while the supplement of polyamides layer, to some extent, enlarge the stress of aluminum alloy. It also suggests that the stress control is related to several key factors - the acoustic impedance, the Poisson's ratio, the ratio of thickness to width of material, the deformation and the wave frequency. [ABSTRACT FROM AUTHOR]

• A failure analysis of the cylinder in a buoyancy regulator of an underwater glider is proposed. • The effects of the ovality on the cylinder sealing performance and health are analyzed. • A failure analysis of the connection bolts in in a buoyancy regulator of an underwater glider is proposed. • The effect of the thickness and size of bolts on the load capacity of the connection bolts are analyzed. The buoyancy regulator is key device used in underwater gliders for adjusting their buoyancy. This paper studies the failures of the cylinder and the connection bolts during the working processing of the buoyancy by using a finite element method. The effect of the ovality on the sealing and the scratching failures of the cylinder is analyzed. Note that the poor machining accuracy should produce the cylinder sealing and the scratching failures. Some suggestions for improving the ideal cylinder design are introduced. Additionally, the specific reasons of the failures of the connecting bolts for the buoyancy regulator are also investigated. Note that the overload causes the failures of the connection bolts. The loading capacity can be increased by making the connecting flange stiffer or by utilizing the larger bolts. The optimization design methods for the piston and the piston rod are provided according to the simulation results. This paper can provide a failure analysis and structural optimization method for the buoyancy regulators. [ABSTRACT FROM AUTHOR]

This work proposed a low symmetry characteristics metamaterial in a single material. The structural configuration of the metamaterial is composed of the reentrant structure, chiral structure and mass inclusion. The wave propagation characteristics are investigated in terms of the Bloch-Floquet wave propagation theorem. Firstly, the band diagram of the metamaterial is analyzed. The wave behaviors of the structure are changed due to the low symmetry characteristics. The irreducible Brillouin zone (IBZ) of the proposed structure is changed. The band diagram in the whole IBZ is analyzed to get the accurate distributions of the bandgaps. Some extremum values of the dispersion surface are deviated from the boundary of the IBZ. The formation mechanisms of bandgaps are illustrated based on the analysis of the vibration mode shapes. Then, the phase velocity and group velocity of the elastic wave are calculated, showing the strong anisotropic properties. Moreover, the effects of the geometrical parameters on the bandgaps are investigated. The locations and width of the bandgaps can be tuned for by the rational design. The existence of bandgaps of the metamaterial are validated by a finite element model. Finally, a metamaterial beam sample with seven cells is made and experiment test are conducted for the vibration attenuation performance and error analysis. The proposed structure can be used for vibration attenuation in the fields of underwater vehicles. • Integrated novel design of reentrant structure, chiral structure and mass inclusion in a single material. • Bloch theorem analysis in the whole irreducible Brillouin zone to find the accurate bandgaps. • Low symmetry characteristics of proposed structure enhanced the anisotropy of the elastic wave. • Design guidance of the full bandgaps by the parametric study. • Finite element model to validate the bandgaps and beam sample are made for error analysis. [ABSTRACT FROM AUTHOR]

The buckling and strength failure of composite cylindrical shells under hydrostatic pressure are studied in this paper by experimental tests and finite element method. Experimental tests were carried out for three composite cylindrical shells with different stacking sequences. In parallel, finite element models are built to predict the buckling and strength failure pressures. The finite element results agree well with the corresponding experimental tests. A parametric study is conducted and the effects of length-radius ratio, thickness-radius ratio, ply angle and stacking sequence on the critical buckling pressure and strength failure pressure are discussed. The results show that length-radius ratio has little influence on the strength failure pressure while thickness-radius ratio, ply angle and stacking sequence have significant influences on the critical buckling pressure and strength failure pressure. The boundary line formed by the intersections between the critical buckling pressure and strength failure pressure of composite shells with same configuration is nonlinear and it is a function of length-radius ratio, thickness-radius ratio, ply angle and stacking sequence. • Buckling & strength failure of composite shells are studied by tests and FE method. • Effects of geometric & ply parameters on buckling & strength failure are analyzed. • Boundary lines between the buckling and strength failure pressures are discussed. [ABSTRACT FROM AUTHOR]

Research on fish locomotion has made extensive progress towards a better understanding of how fish control their flexible body and fin for propulsion and maneuvering. Although the biologically flexible fish fins are believed to be one of the most important features to achieve optimal swimming performance, due to the limitations of the existing numerical modeling tool, studies on a deformable fin with a non-uniformly distributed stiffness are rare. In this work, we present a fully coupled fluid–structure interaction solver which can cope with the dynamic interplay between flexible aquatic animal and the ambient medium. In this tool, the fluid is resolved by solving Navier–Stokes equations based on the finite volume method with a multi-block grid system. The solid dynamics is solved by a nonlinear finite element method. A sophisticated improved IQN-ILS coupling algorithm is employed to stabilize solution and accelerate convergence. To demonstrate the capability of the developed Fluid–Structure-Interaction solver, we investigated the effect of five different stiffness distributions on the propulsive performance of a caudal peduncle-fin model. It is shown that with a non-uniformly distributed stiffness along the surface of the caudal fin, we are able to replicate similar real fish fin deformation. Consistent with the experimental observations, our numerical results also indicate that the fin with a cupping stiffness profile generates the largest thrust and efficiency whereas a heterocercal flexible fin yields the least propulsion performance but has the best maneuverability. [ABSTRACT FROM AUTHOR]