某水下发射装置自由弯曲振动的流固耦合分析(英文)

A pneumatic launcher is theoretically investigated to study its natural transverse vibration in water. Considering the mass effect of the sealing cover, the launcher is simplified as a uniform cantilever beam with a top point mass. By introducing the boundary and continuity conditions into the motion equation, the natural frequency equation and the mode shape function are derived. An iterative calculation method for added mass is also presented using the velocity potential function to account for the mass effect of the fluid on the launcher. The first 2 order natural frequencies and mode shapes are discussed in external flow fields and both external and internal flow fields. The results show good agreement with both natural frequencies and mode shapes between the theoretical analysis and the FEM studies. Also, the added mass is found to decrease with the increase of the mode shape orders of the launcher.And because of the larger added mass in both the external and internal flow fields than that in only the external flow field, the corresponding natural frequencies of the former are relatively smaller.     

一维弹性结构在理想流体中的模态特性研究

为研究一维弹性结构在理想流体中的模态特性,建立了流固耦合振动模型。基于流体无旋、无粘不可压缩的假设,求解流体域内的速度势,根据流场分析得到水压力,将脉动水压力加载在结构表面,得到结构在水中的振动方程,并利用瑞利-利兹法求解得到圆柱梁在水中振动的固有频率和振型。在此基础上,基于应变测试对水池中的悬臂梁结构进行了模态识别,实验结果与计算结果相当吻合。研究发现,由于附连水质量的影响,弹性结构在水中的固有频率低于其在空气中的,其振型与空气中的形状基本一致,但其节点会向自由端偏移。     

水下结构声固耦合振动的特征值计算

针对水下结构声固耦合振动计算中出现的非对称矩阵(与频率相关)的特征值问题,本文基于共轭子空间迭代法,提出了计算水下结构振动固有频率和振型的双重迭代算法.数值算例表明:该算法收敛性好,用于水下结构声固耦合振动的特征值计算是有效的.文中分别将流体视为不可压缩和可压缩两种情况建立了流固耦合方程,计算了板、加筋板和方箱振动的固有频率和振型,将干模态和湿模态、不可压缩流体和可压缩流体的计算结果进行了比较,并分析了结构刚度和流体边界条件对附加水质量影响系数的影响.     

浅水域结构流固耦合振动的固有频率计算

本文首先建立了浅水域声学边界元方程和相应的FEM/BEM耦合振动方程,探讨了水深对结构振动固有频率和振型的影响,流体可压缩性对结构振动固有频率的影响.研究结果表明:水深变化对结构固有频率有影响,随着水深减小,结构固有频率降低,特别是当水深与结构浸深接近时,水深变化对固有频率影响极大,且振型顺序也可能发生变化;而流体的可压缩性对于浅水域结构固有频率影响不大.     

海洋立管悬挂状态的固有频率和振型

文章把悬挂状态的立管简化为一端固定一端自由的梁模型,考虑截面性质和有效张力及质量的变化,其控制方程为变系数的四阶偏微分方程,运用微分变换方法推导了固有频率和振型的数值计算方法。通过数值算例验证了文中方法的有效性,考察了收敛速度的变化情况,讨论了立管末端的悬挂器具对固有频率和振型的影响,并结合工程实际,给出了若干海况下立管悬挂状态时在飞溅区的共振区域。     

Sensitivity analysis and dynamic modification of modal parameter in mechanical transmission system

Sensitivity analysis is one of the effective methods in the dynamic modification. The sensitivity of the modal parameters such as the natural frequencies and mode shapes in undamped free vibration of mechanical transmission system is analyzed in this paper. In particular,the sensitivities of the modal parameters to physical parameters of shaft system such as the inertia and stiffness are given. A calculation formula for dynamic modification is presented based on the analysis of modal parameter. With a mechanical transmission system as an example, the sensitivities of natural frequencies and modes shape are calculated and analyzed. Furthermore, the dynamic modification is also carried out and a good result is obtained.     

Analytical and experimental study of free vibration of beams carrying multiple masses and springs

The structures in engineering can be simplified intoelastic beams with concentrated masses and elastic spring supports.Studying the law of vibration of the beams can be a help in guidingits design and avoiding resonance. Based on the Laplace transformmethod, the mode shape functions and the frequency equations ofthe beams in the typical boundary conditions are derived. Acantilever beam with a lumped mass and a spring is selected toobtain its natural frequencies and mode shape functions. Anexperiment was conducted in order to get the modal parameters ofthe beam based on the NExT-ERA method. By comparing theanalytical and experimental results, the effects of the locations ofthe mass and spring on the modal parameter are discussed. Thevariation of the natural frequencies was obtained with the changingstiffness coefficient and mass coefficient, respectively. The findingsprovide a reference for the vibration analysis methods and thelumped parameters layout design of elastic beams used inengineering.     

简支梁损伤定位的直接指标法应用

结构损伤诊断主要内容是对各种工程结构进行检测并对检测结果做适当的分析,从而确定结构的健康状况。损伤诊断要解决的关键问题之一是判断结构损伤的位置。结构损伤会对结构的物理特性如刚度,质量,阻尼等产生影响,从而使结构的模态参数如固有频率、振型等发生改变。研究结果表明应变相比固有频率和振型对局部损伤更加敏感。基于应变模态差分原理的损伤定位直接指标法ISMSD(Strain Mode Shape Difference)具有无需原始模态数据的优点。本文通过有限元方法计算得到非贯穿裂纹梁的位移模态,进而计算得到应变模态。再对经过三次样条插值的光滑应变模态差分曲线进行直接损伤指标值计算。对假定两处损伤的简支梁在几种不同损伤程度状况下进行数值仿真计算,直接根据直接指标值的大小正确地判定了损伤位置,尤其是损伤量较小时。     

边界任意弹性约束薄板的横向振动(英文)

This paper presents an analytical study on the influence of edge restraining stiffness on the transverse vibrations of rectangular plate structure. An improved Fourier series method was employed to analyze the transverse vibration of plate structure with general elastically restrained boundary conditions. A linear combination of a double Fourier series and eight auxiliary terms was sought as the admissible function of the flexural displacement of the plate, each term being a combination of a polynomial function and a single cosine series expansion. The auxiliary terms were introduced to ensure and improve the smoothness of the original displacement function and its derivatives at the boundaries. Several numerical examples were given to demonstrate the validity and accuracy of the current solution. The influences of translational and rotational stiffness on the natural frequencies and mode shapes of plate were analyzed by numerical results. The results show that the translational stiffness has bigger influence on the natural frequencies than the rotational stiffness. It is generally well known that little change of the rotational stiffness has little influence on the mode shapes of plate. However, the current work shows that a very little change of rotational stiffness value may lead to a large change of the mode shapes of a square plate structure.     

Mode transitions in vortex-induced vibrations of a flexible pipe near plane boundary

A pipe model with a mass ratio（mass/displaced mass） of 4.30 was tested to investigate the vortex-induced vibrations of submarine pipeline spans near the seabed.The pipe model was designed as a bending stiffness-dominated beam.The gap ratios（gap to diameter ratio） at the pipe ends were 4.0,6.0,and 8.0.The flow velocity was systematically varied in the 0-16.71 nondimensional velocity range based on the first natural frequency.The mode transition between the first and the second mode as the flow velocity increases was investigated.At various transition flow velocities,the research indicates that the peak frequencies with respect to displacement are not identical along the pipe,nor the frequencies associated with the peak of the amplitude spectra for the first four modes as well.The mode transition is associated with a continuous change in the amplitude,but there＇s a jump in frequency,and a gradual process along the pipe length.     

一种改进的海洋平台结构损伤诊断方法研究(英文)

In the exploitation of ocean oil and gas, many offshore structures may be damaged due to the severe environment, so an effective method of diagnosing structural damage is urgently needed to locate the damage and evaluate its severity. Genetic algorithms have become some of the most important global optimization tools and been widely used in many fields in recent years because of their simple operation and strong robustness. Based on the natural frequencies and mode shapes of the structure, the damage diagnosis of a jacket offshore platform is attributed to an optimization problem and studied by using a genetic algorithm. According to the principle that the structural stiffness of a certain direction can be greatly affected only when the brace bar in the corresponding direction is damaged, an improved objective function was proposed in this paper targeting measurement noise and the characteristics of modal identification for offshore platforms. This function can be used as fitness function of a genetic algorithm, and both numerical simulation and physical model test results show that the improved method may locate the structural damage and evaluate the severity of a jacket offshore platform satisfactorily while improving the robustness of evolutionary searching and the reliability of damage diagnosis.     

Effect of internal flow on vortex-induced vibration dynamics of a flexible mining riser in external shear current

Vortex-induced vibration (VIV) of flexible risers with both internal and external flows has received much attention recently. Hence, VIV dynamics of a fluid-conveying flexible riser subjected to external shear current is investigated. The effect of internal flow velocity and fluid density on VIV response is mainly examined and analyzed. A time domain model is introduced and elaborated. Then the finite element method is adopted to discretize the governing equations. Firstly, the model is validated based on the comparison between the numerical and experimental results. Then the influence of the internal flow velocity and fluid density on VIV dynamics is studied. The results show that multi-frequency response occurs when the flexible riser with various internal flow velocities and densities is subjected to external shear current. Under same shear current velocity, the IL mean deflection is enlarged with the increase of the internal flow velocity and fluid density. In addition, the internal flow velocity and fluid density have an evident effect on the vibrating frequency and the root mean square (RMS) displacement in both in-line (IL) and cross-flow (CF) directions. Moreover, mode and frequency transitions can be observed under different internal flow velocities and fluid densities.     

考虑流体静压时充液圆柱壳的输入能量流特性

研究了内部流体静压力对充液圆柱壳的受迫振动能量流的影响。静压力作为预应力计入壳体振动方程当中,壳体和流体分别采用Fliigge方程和Helmholtz波动方程。分析了耦合系统在周向线分布余弦载荷下的动态响应．用傅立叶变换和反变换实现空间域与波数域之间的相互转换,采用留数定理求取系统的响应并得到耦合系统的输入能量流。结果表明,周向模态数n=0时,静压力对耦合系统的输入能量流影响很小;n较大时,在中低频下静压力使输入能量流曲线沿频率轴右移,由于输入能量流的峰值点处的频率对应于频散曲线中各支传播波的截止频率,对应传播波的截止频率升高,高频时则基本无影响。随着静压力以及周向模态数的增大,输入能量流受到的影响程度也增大。     

带流水孔潜体流场数值模拟

本文通过求解RANS方程,结合PISO算法与k-ω湍流模型,数值模拟了带有两种不同形式流水孔(纵缝式与格栅式)潜体的内外流场.数值模拟给出了流水孔流场的精细结构,计算并分析了潜体各个壁面上阻力成分的大小与成因.计算得到的流水孔阻力增量与试验值吻合较好.对计算结果的分析表明:流水孔引起的阻力增量主要集中在导流板与流水孔壁上;内部空腔壁上的阻力值是可以忽略的小量;涡旋流动主要集中在导流板之间;流水孔阻力系数随雷诺数的增加而趋于一稳定值.计算结果表明,本文中使用的方法可用于模拟流水孔内外流场.     

基于BEM/FEM计算浸水结构振动特性

应用直接边界元和有限元方法分析水下结构体的固有频率和振动特性,根据边界元理论编译出相应的程序来计算水下结构的附加质量矩阵,将附加质量矩阵与现有的有限元程序相结合,分析水下结构体的固有频率和振动特性,以浸没于流体介质中的圆柱壳为例,通过与近似理论值的对比验证了数值计算结果的可靠性,表明流体介质改变了结构的振动特性.     

Analysis of the hull girder vibration by dynamic stiffness matrix method

1 Introduction1 The excitations of the main engine and propellers may induce harmful vibration of the ship. To avoid resonance, the overall natural frequencies and responses of the ship hull must be calculated firstly, and the results can be taken as the …     

Calculation method of underwater acoustic and vibration response of cabin segment based on onshore vibration test北大核心CSCD

[Objective]This paper proposes a method for calculating the acoustic and vibration response of underwater cylindrical shell structures based on land-based vibration test results.[Methods]An axisymmetric boundary element method (BEM) is introduced to describe the radiation acoustic field of the shell. The relationship of acoustic pressure at the nodal point of the generatrix with velocity is obtained by solving the numerical solution of the boundary element integral equation, then the acoustic radiation impedance matrix of the outer surface of the shell and acoustic transfer vector (ATV) are constructed. Based on the assumption that the low-order vibration mode of an underwater cylindrical shell is the same as that of an onshore cylindrical shell, combined with the modes and acoustic radiation impedance matrix of an onshore cylindrical shell, the modal added mass and damping are calculated. [Results]The natural frequency calculation formula of the underwater cylindrical shell is established on the basis of the onshore mode, and the calculation method of underwater vibration response and acoustic radiation characteristics with the vibration response in air as input is obtained based on mode superposition method.[Conclusion ] The numerical results of a typical cylindrical shell with internal structure show that the method meets the engineering accuracy requirements. © 2023 Chinese Journal of Ship Research. All rights reserved.     

龙门起重机结构模态分析

分析JMQ50型龙门起重机结构组成,采用有限方法并用ANSYS软件对整机结构进行建模并进行模态分析,得出各阶固有频率和振型,为设计提供理论依据。     

Validation and application of nonlinear hydrodynamics from CFD in an engineering model of a semi-submersible floating wind turbine

Nonlinear hydrodynamics play a significant role in accurate prediction of the dynamic responses of floating wind turbines (FWTs), especially near the resonance frequencies. This study investigates the use of computational fluid dynamics (CFD) simulations to improve an engineering model (based on potential flow theory with Morison-type drag) by modifying the second-order difference-frequency quadratic transfer functions (QTFs) and frequency-dependent added mass and damping for a semi-submersible FWT. The results from the original and modified engineering models are compared to experimental data from decay tests and irregular wave tests. In general, the CFD results based on forced oscillation tests suggest increasing the frequency-depending added mass and damping at low frequencies compared to first order potential flow theory. The modified engineering model predicts natural periods close to the experimental results in decay tests (within 5%), and the underprediction of the damping is reduced compared to the original engineering model. The motions, mooring line tensions and tower-base loads in the low-frequency response to an irregular wave are underestimated using the original engineering model. The additional linear damping increases this underestimation, while the modified QTFs based on CFD simulations of a fixed floater in bichromatic waves result in larger difference-frequency wave loads. The combined modifications give improved agreement with experimental data in terms of damage equivalent loads for the mooring lines and tower base.     

通过形状差异修正模式方法进行的损伤识别试验(英文)

An experimental program was undertaken to test the feasibility to detect the occurrence of structural damage using a modified mode shape difference technique. The vibration response of a steel beam fixed at one end and hinged at the other was obtained for the intact and damage conditions. Modal analysis was performed to extract the frequencies and mode shapes. The method shows a good potential in detection of occurrence and location of damage.