During the deployment and recovery phases of the ROV, vessel heave caused by complex sea conditions may induce violent oscillations, leading to collisions between the ROV and the moon pool, posing a threat to structural safety. Given the growing use of ROVs in offshore operations, ensuring their stability is crucial for both operational safety and efficiency. To address this issue, this study introduces a novel nonlinear pendulum model that incorporates both vessel heave motion and hydrodynamic effects, providing new insights into the impact of vessel heave on ROV dynamics. Using finite element simulations, the study identifies unsteady parametric resonance in the ROV's swing response, characterized by a time-varying swing amplitude, which is influenced by the displacement phase difference (parametric excitation phase) between the vessel and the ROV. Analysis with the multiple-scale method and Floquet theory reveals that both amplification and attenuation effects coexist in the parametric resonance response of the ROV. The variation in the parametric excitation phase reflects the competition between these effects and determines the divergence rate of the ROV's swing response. Due to nonlinearity and other factors, the parametric excitation phase evolves over time. When the system stabilizes, the phase converges to -pi/2, deviating from conventional parametric excitation problems where the phase is typically +/-pi. This study also explores the steady-state convergence of the excitation phase through a negative feedback mechanism of nonlinear damping, providing a deeper understanding of phase behavior in nonlinear parametric resonance.

With the increasing demands of higher performance of advanced aircrafts, the flexible sealing have been used to seal the gap between main wing surface and its movable control surface so as to improve stealth performance and control effectiveness. However, during whole flight process undergoing various operation states and severe loadings, the flexible sealing could be separated from control surface and consequently strongly vibrate under the disturbances coming from ambient airflow. In that case, "flapping" motions between flexible sealing and other wing components may cause complicated nonlinear dynamic structural response due to discontinuous boundary conditions along with impact effect. In this study, the static structural responses in multiple states, including original state, installation state and lifting process, are firstly examined based on FEM numerical simulations. The displacements and contact preloads that can influence the dynamic characteristics and responses, changing with flight states and speeds, are given so as to analyze and model the discontinuous boundary conditions. Furthermore, the dynamic responses of the flexible sealing during its flapping motions under these particular discontinuous constraints, considering combination actions of periodic excitation and intermittent impact forces, are comprehensively studied. Our numerical results show that the dynamic displacement amplitude and root moment of flapping motion increase respectively by 86.3 % and 177.9 % than static values. And, a mixing of standing waves and traveling waves of the acceleration and shear stress responses is observed through spatiotemporal evolutions. Generally speaking, the flapping is a dynamic response with broadband spectrum rather than a simple forced vibration, which include superharmonic frequencies of excitation frequency, natural frequency and high frequencies due to impact. More interestingly, significant nonlinear phenomena such as superharmonic resonance and chaos are found due to combination actions of discontinuous constraints and intermittent impacts. To deeply explore the nonlinear behaviors of this flapping motion, an analytical model including stiffness jump and impact force is developed, and the Runge-Kutta algorithm is used to obtain the solutions of the nonlinear system. The phase diagram and Poincare section of the analytical solutions give similar qualitative results with our numerical simulations.

With the increasing complexity of aircraft’s wing structures, it becomes really important and more challenging to study the response of movable components under dynamic loads and the interactions between various components. For example, functional composite skins, used for aerodynamic sealing of the gap between the main wing surface and the movable control surface, could be separated from the control surface under the disturbances coming from ambient aerodynamic forces, and it could vibrate under the action of vortex shedding in its wake flow field. Sometimes, these complicated excitations can cause “flapping” motions between wing components, and consequently it can strongly affect the safety of the wing structure. Static contact analysis and dynamic response analysis of functional composite skin under nonlinear contact boundary conditions are conducted in this study based on FEM numerical simulations. The changes of internal stress and boundary contact force of the skin during multiple processes are studied. Further more, a simplified two-dimensional beam model is used to investigate the spatial-temporal evolutions of displacement of functional composite skin during the “flapping” process. The results show that, owing to the “flapping” effect, the displacement amplitude and boundary contact force of functional composite skin significantly increase. Compared with static contact condition, the displacement amplitude increases by about 86.25

Bionic flapping hydrofoil motion is increasingly used in bionic underwater robots. However, many scholars have focused their attention on the navigation problem in deep-water environments, thus ignoring changes in the hydrodynamics of hydrofoils under the action of the near-free-surface effect. This paper studies and explores the hydrofoil motion in near-free surfaces through the RANS viscous flow numerical simulation method, combined with overset mesh and adaptive mesh technology. Three different forms of motion are studied respectively, including a stationary fixed, a single-degree-of-freedom pitch and a two-degree-of-freedom heaving-pitching coupled hydrofoil. The effects of varying the immersion depth d on the lift and thrust generated were analyzed. Results indicate noticeable differences in the free surface action among different motion forms. When the water depth is less than one chord length C, the lift and thrust of the three motion forms decreased rapidly decrease. When d/C=1~1.5, the static fixed hydrofoil lift and thrust gradually approach the deep-water state. When d/C>2, the pitching motion of a single degree of freedom also tends to be stable. The two-degree-of-freedom motion is d/C>3. This finding shows that the effect of the near-free surface is closely related to the vertical motion. The greater the vertical motion is, the more severe the effect.

Functional composite skin has been increasingly used in advanced flight vehicles and some industrial complex systems. For example, it is usually used as aerodynamic sealing structure of the gap between main wing surface and movable control surface so as to ensure the continuity of the wing surface, which can effectively improve flight performance and stealth performance of flight vehicles. The dynamic characteristics analysis of functional skin is of great significance for its reliability and stability design. In this study, the dynamic characteristics of functional composite skin under different installation and flight states, including the original state (non-contact state), the installation state (considering nonlinear contact boundaries), and the working state (considering both nonlinear contact boundary and pressured inner surface), are analyzed based on FEM numerical simulations. The results indicate that: unlike the two-dimensional uniform plates, new “bending-torsion-coupling” modal shape appears in the composite laminates. The modal frequencies and modal shapes of functional skin can be mainly divided into two kinds, i.e. the contact-modes and non-contact-modes, while various non-linear contact conditions are considered, which provides a guide to further dynamic, and particularly aeroelastic, analysis of such structures during structural configuration design and strength assessment. And, their modal behaviors in terms of chordwise and spanwise characteristics are significantly different. Moreover, the contact states and modal shapes are affected by flight velocity, and, even in some states, the dynamic characteristics will undergo substantial changes principally because of the different contact states.

Flexible structures with point buoyancy widely exist in nature and engineering. Under the action of oscillating flow, it usually has a large geometric nonlinear dynamic response. However, the dynamic response and drag reduction of flexible structures with point buoyancy have not been studied. Therefore, the numerical method in this paper investigates the dynamic response and drag reduction of point buoyant flexible structures under oscillatory flow. Firstly, complex spatial curvilinear coordinates establish the dynamic partial differential equations of flexible structures with point buoyancy. Then, the implicit finite-difference time-domain method is used to discretize the partial differential equation in space and time to form an algebraic equation. Finally, the dynamic response and drag reduction of flexible structures under non-buoyancy, uniform buoyancy, and point buoyancy are numerically analyzed. The results show that with the increase of Cauchy number CY, the deformation of the flexible structure becomes larger and larger, and a local bending point appears. The dimensionless vibration frequency numbers explain the occurrence of local bending points. Unlike no buoyancy, uniform buoyancy and point buoyancy make the flexible structure smaller and more symmetrical. Uniform buoyancy and point buoyancy can increase the Reconfiguration number R. The greater the buoyancy and buoyancy position, the greater the Reconfiguration number R. The load on the flexible structure under oscillating flow is still less than that on the rigid structure. The Vogel exponent is calculated by fitting the Reconfiguration number R. The drag reduction is directly proportional to the Vogel exponent v, that is, the greater the Vogel exponent v, the greater the drag reduction. When the Cauchy number CY is large, the Vogel exponent v of uniform buoyancy and point buoyancy is smaller than that of non-buoyancy. The greater the point buoyancy and buoyancy position, the smaller the deformation of the flexible structure, the greater the Reconfiguration number R, and the greater the Vogel exponent v. When the buoyancy position is small, the influence of point buoyancy on the flexible structure can be ignored.

The acoustic emission (AE) technology is commonly used for detecting cracks in aircrafts, and the moment tensor inversion is an advanced signal post-processing methodology for quantitatively calculating fracture mechanism. The inversion accuracy of the traditional inversion approach is significantly dependent on precisely recorded waveforms, which require high sampling frequencies of signal recording. For reducing the requirement of sampling frequencies, an extended inversion approach for moment tensors is proposed and the accumulated energy of the signals is used to invert for fracture mechanism. The synthetic tests show that the inversion accuracy of the new inversion approach is more stable for various sampling frequencies than that of the traditional displacement-based one. The inversion results calculated by the new approach are less sensitive to noise, and accurate results can be achieved by less sensors. Generally, the extended inversion approach can help evolve the structural health analysis of aircrafts significantly.

Due to the interactions between ocean environmental loads, remotely operated vehicle (ROV), suspending cable and top-end vessel motions, the dynamic responses of ROV system have strong coupling and nonlinear characteristics. Firstly, the FEM numerical simulations of ROV system, which include the vessel surge and the hydrodynamic force related to ROV motion, are developed to examine dynamic responses of ROV system. It is found that, the ROV response can be amplified when frequency ratio is larger than 0.7, up to 5.95 times of the vessel surge amplitude. As the surge amplitude increases, the ROV displacement gets larger. However, the response amplification factor (RAF) becomes smaller owing to nonlinear damping coming from hydrodynamic force, particularly when the frequency ratio is close to 1.0. Furthermore, to solve the nonlinear governing equation of the ROV dynamics, an equivalent damping model is developed based on ROV displacement attenuation, and a semi-analytical expression as a function of the ROV displacement along with surge amplitude and frequency, is obtained. And the method of multiple scales is employed to obtain the asymptotic solution. It is seen that in addition to the primary resonance at the frequency ratio of 1, secondary resonances occur at frequency ratios of 1/3 and 3. Finally, the implicit and explicit formulas of the RAF, depending on the excitation frequency, amplitude and the developed equivalent damping, are given respectively, so that we could have a deeper insight into the nonlinear behaviors of the ROV response.

Due to the complexity of the integrated Floating Wind Turbine (FWT) system, obtaining reliable results necessitates extensive experiments. This paper conducts a comprehensive study on the motion performance and mooring load responses of a novel 12-MW semi-submersible FWT through model tests carried out in a wave basin. A multi-blade large-scale wind-generation system, equipped with a rectifier network, was enhanced and constructed to provide a dependable wind field. And a flexible tower was designed and fabricated, achieving an accurate simulation of the tower's stiffness characteristic and its impact on the overall dynamic response. The marine environmental conditions encompass various combinations of wind, waves, and currents. Rigorous calibration and identification tests were undertaken to validate the environmental conditions and the model system. The findings reveal that, under mild wave parameters, the mooring load is primarily influenced by the resonance response with platform motions, particularly surge resonance. The load effect of wind and current induces mean surge and pitch motions, while their damping effect reduces the standard deviation of responses, notably suppressing the pitch response peak at its natural motion frequency. Wave loads predominantly dictate the vibration range of motion responses. When the current velocity reaches a sufficient magnitude, the coupling effect between current and wave in the wave-frequency region significantly amplifies the mooring response. Notably, motions and mooring loads in the 60-deg and 90-deg directions surpass those in the 0-deg direction, with the maximum responses occurring at 60 deg.

Remotely operated vehicle (ROV) system is significantly affected by the excitations coming from the top -end vessel motion and the environmental hydrodynamic forces. Especially, during initial deployment and final recovery stages, the relative motion between suspending ROV and moving top -end vessel gets more complicated and even larger. This may cause a body collision, and consequently the cable tension can also be severely changed by larger ROV motion. Sometimes, cable slack may occur, which seriously affects structural safety. To study the dynamic responses of ROV system induced by vessel heave during initial deployment and final recovery processes, the FEM numerical simulations, combining with the hydrodynamic model are conducted. Then the ROV displacement and cable tension time histories are presented. The influences of vessel motion amplitude and frequency, along with initial condition, on the dynamic responses of ROV system are also discussed. Moreover, to explore the dynamic behaviors of the ROV system further, we discuss the physical mechanisms driving response amplification through theoretical models. The governing equation of ROV system dynamics with a vertical moving boundary is established, and a dimensionless control parameter called amplitude -frequency factor, which characterizes the dynamic excitation effect of vessel heave, is proposed. Then, in order to study the influences of nonlinear hydrodynamic damping effect on ROV response, an empirical formula for hydrodynamic damping ratio is proposed based on ROV displacement time histories. Finally, under consideration of the nonlinear damping effect, the unstable regions of vessel motion amplitude and frequency at different initial conditions are obtained. The results show that the vessel heave may induce a parametric excitation response to the ROV system. Particularly, parametric resonance is most pronounced, when the vessel motion frequency is close to twice the natural frequency of the ROV system. While, under the nonlinear damping effect generated by hydrodynamic force, the parametric resonance occurs when the amplitude -frequency factor reaches its threshold. The unstable regions under hydrodynamic damping are smaller compared with that without damping. Furthermore, as the initial angular displacement of ROV increases, the unstable region becomes smaller further, or, in other words, the parametric conditions of vessel heave for ROV response amplification become more demanding.

Remotely operated vehicles (ROVs) are frequently used in subsea explorations and exploitations, aiming to obtain not only traditional ocean oil and gas, but also other mineral resources, such as manganese, polymetallic nodules and sulfides. However, due to the interactions between ocean environmental loads, ROV, suspending cable and top-end vessel, the dynamic responses of ROV system have strong coupling and nonlinear characteristics. Especially, during the initial deployment and the final recovery stages, the cable length might be so short that the relative motion between the ROV and the vessel may cause a severe collision and even structural damage. In this study, based on the structural and dynamic characteristics of the ROV system during the initial deployment and the final recovery stages, numerical simulation and analytical method are employed to investigate the impacts of the top-end vessel motion on the ROV system. In the numerical simulations, vessel surge and hydrodynamic force on ROV, are considered, and the ROV responses and cable tensions under different vessel motion amplitudes and frequencies are presented. It is found that the ROV responses are larger than the vessel motion amplitudes in some cases owing to vessel surge. And, the governing equation of ROV with a horizontal moving boundary is developed. Then, in order to have a deeper understand of the behaviors of the ROV system and the mechanisms of our simulation results, the analytical method is used.

In nature and engineering, a uniform flexible beam with point buoyancy is common, such as water lily and stem, kelp, buoy and mooring, and deep-sea flexible riser. Response to a flexible beam with point buoyancy is more complicated than that of a flexible beam. Therefore, this paper conducts theoretical numerical and water tank drag experimental research on the drag reduction problem of flexible beams with point buoyancy. First, a governing equation with point buoyancy is established. Then, an explicit iterative numerical method is proposed to solve large geometric nonlinear differential equations. Finally, numerical and experimental methods studied the drag reduction phenomenon of flexible beams with point buoyancy. The results show that the reconfiguration shape of flexible beam with point buoyancy is no longer self-similar, and there are locally bending points in beam deformations. It is found that the Vogel exponent curve fluctuates due to the non-self-similarity of the deformation shape. Moreover, as the buoyancy gets more excellent, the fluctuation range of Vogel values becomes more profound.

As the exploration and exploitation of deep-sea oil and gas, along with promising polymetallic nodule&sulfides mining, have been developing toward ultra-deep waters, some innovative concepts of marine cable configuration suitable for ultra-deepwater are proposed, such as stepped cable, hybrid cable and double-stepped cable. For deep-water cables with complex configurations, the structural responses become more complicated due to their non-uniform structural properties. Because the distributed buoyancy modules along cable length might introduce more significant local bending segments. Moreover, the impacts of moving boundary, caused by the motions of top vessel and bottom mining vehicle, should be considered. Through combing the finite element simulations with the hydrodynamic models, the dynamic response analysis approach of ultra-deepwater cables is established in this study. Then the double-stepped cable responses, including axial tension, displacement along with the change of overall configurations caused by moving top vessel and bottom mining vehicle, are calculated. Moreover, wave propagation behaviors during cable response are comprehensively examined, and the influences of non-uniform structural properties on cable response and wave propagation are analyzed using the wave propagation theory of structure with axially varying properties based on the Bessel function. The results show that the presented double-stepped cable can provide suitable configurations during the dynamic response, which has good compliance performance and can effectively buffer its response caused by moving boundary excitation. Finally, we found that the response spatial-temporal evolutions present some interesting phenomena, such as the axially non-uniform characteristics lead to non-monotonic changes in response amplitude and wavelength, with local peaks occurring in the low-tension region, owing to the distributed buoyancy modules, along with axially-varying and discontinuous structural properties. And, there exists significant mixed effect coming from both standing waves and traveling waves.

The knowledge of cracking mechanisms is significant for evaluating the healthy condition of aircraft structures and can be retrieved by moment tensor inversion based on the acoustic emission (AE) phenomenon. For engineering applications, the inversion method cannot compute accurate results because the waveforms recorded by sensors are commonly contaminated by noise. Consequently, the correlation calculation of de-noising is introduced into the inversion and sufficient correlation functions are needed. In this paper, the correlation function of raw waveforms is proposed and based on the inherent similarity between the signals induced by one source and recorded by different sensors. According to the synthetic tests, the error of the inversion method based on the new correlation function is approximately 1/10 of that of the commonly used amplitude method. Although the inversion accuracy is influenced by the phase differences and the ratio of noise frequency to signal frequency, the influence is limited and the new correlation function is suitable for most practical cases. The inversion method based on the new correlation function does not require the knowledge of noise spectra or any complex calculation processes and provides a new way to improve the inversion accuracy of cracking mechanisms with little additional computation consumption.

实验实践类教学是专业培养计划的重要组成部分,相对于理论课程,理论课程配套实验或实验实践类课程在师资配置、教学方法与课程内容方面更需提升.对于教学实验内容与要求的设计,目前存在流程固化和测试数据为本的趋势.教学实验的目的和作用是教学内容设计与评价标准设定的基础,这也是"回归初心"的体现.本文通过材料力学实验课程现状分析,探讨教学实验课程的内容设计与实施模式.

Hydraulic fracturing and induced networks are significant for effective production of oil and gas from unconventional resources. The knowledge of fracturing source mechanisms is helpful for optimizing hydraulic fracturing treatments to maximize production. For source monitoring, the moment tensor inversion is commonly used and the source mechanisms are interpreted by the radiation patterns of microseismic waves. The accuracy of source interpretation is significantly influenced by sensor configurations, which still need further researches. In this study, the mechanism of sensor arrangements to suppress noise effect is analyzed and clarified mathematically, then an optimization method of searching for proper sensor configurations is proposed. For superior sensor configurations, errors caused by noise can be allocated evenly to the 6 moment tensor components and the source interpretation based on the moment tensor decomposition is till accurate, but errors can not be completely eliminated by optimizing sensor configurations. Generally, high-precision inversion results can be calculated by the sensor configuration that one sensor is at the center and the others are around at the same angular intervals. Compared with tradition sensor configurations, this new one can achieve similar inversion accuracy by less than a third of the sensors. Sensor numbers are not the more the better and dependent on the sizes of the regions of sensor arrangement. The conclusions arrived in this study are helpful for evaluating and designing sensor configurations for hydraulic fracturing monitoring.

深水柔性缆线是深海资源开采系统的重要组成部分,随着水深的增加,深水缆线长径比达103,结构柔性变得很大,且沿展向非均匀分布的浮力模块,使得缆线构型变得更加复杂、张力等结构参数沿展向变化,这使得环境载荷作用、海面船体运动等激励作用下的缆线流固耦合响应变得更加复杂,给结构安全带来严峻挑战.本文针对Double-stepped这种新构型深水缆线,基于其流固耦合特性和载荷模型表征,建立了含分布浮体的深水缆线动力学控制方程,并结合有限元数值模拟和水箱模型实验,进行了复杂构型缆线的动响应研究.考察规则波和极端波浪环境载荷作用、海面船体运动等激励因素对结构响应的影响,给出位移和张力等响应的时空演化规律,并基于WKB理论分析了变参数结构响应幅值和波长的演化规律和机理.结果表明:结构响应沿着缆线长度向下传播过程中非单调变化,在低张力区域会出现局部峰值;由于分布浮体的存在使得结构参数轴向变化且不连续,响应时空演化变得更加复杂,呈现驻波、行波混合效应,而且张力不但会影响位移幅值,还会引起响应传播过程中的波长改变.

The ANN model trained on experimental datasets is developed, especially based on the characteristics of flexible cylinder's VIV, the Bayesian regularization back propagation algorithm is employed to train the presented neural network. Nine intuitive physical parameters are selected according to the governing equations of cylinder dynamics. The results show that the neural network trained with intuitive physical quantity can acceptable predictions predict VIV, and the linear regression value is 0.940. In addition, the range of model parameters is limited in the trained neural network, with around 20% error.

平面应力(应变)转轴公式是材料力学应力(应变)分析部分的主要教学内容,尽管转轴公式基于特定的应力(应变)状态导出,但在后续的应用,包括例题与习题中的引用并没有限定于平面应力(应变)状态.为了澄清学生常见疑惑,本文使用简单的方式对该问题进行说明,供教师教学时参考.

The dynamic response of a 5 MW floating wind turbine is examined using the coupled finite element simulations, and the dynamic effects of the catenary is considered, while the coupling between flexible components are included. The restoring performance of the mooring-line is analysed based on the vector equations and numerical simulations. The stiffness hysteresis and the influence of the catenary dynamics on the restoring performance are studied. Then the structural responses undergoing wind and wave loads, are examined. Our results show that the mooring-line tension significantly rise due to catenary dynamics, and the snap tension gets around three times larger. The mooring-line stiffness presents a hysteresis character, owing to the fluid/structural damping, which becomes more obvious with the increase of motion frequency/amplitude. Moreover, the structural response gets smaller principally because of the hysteresis effect. The spar displacement and the tower root stress become respectively 18.4% and 32.7% smaller.