Implementation of active control on flexibly mounted pipe exposed to vortex induced vibration using rotating rod

This paper presents the experimental works on the active control implementation of a flexibly mounted cylinder exhibiting vortex induced vibration phenomena. The dynamic response of the system was initially studied by fixing single and double passive control rods to the main cylinder. It was found that the fixed control rod could not suppress the vibration for both cases. In fact, the vibration was amplified as compared with the bare cylinder response. By retaining the same position, an active open loop control strategy was introduced to the vibrating system by rotating the single control rod in clockwise and counter clockwise directions. The results indicated that the counter clockwise direction has suppressed the vibration up to 29.81 dB. Further investigation was made by studying the performance of the control rod at several speeds in a single reduced speed of 4.29. It was found that the maximum attenuation level of 19.56 dB was attained based on 1676 RPM of control rod rotating speed. In addition, an overview of attenuation level trends at both varying control speed and reduced speed were presented in this study. Through the experimental implementation of control strategies conducted in this research, it was found that the active control strategy has performed well as suppression strategy at ineffective fixed control rod position.     

紫藤萝复叶气动特性的风洞实验研究

树叶的形状重构和减阻能力在太阳能帆板、机翼结构、仿生天线设计和新型发电技术等方面具有应用价值.紫藤萝羽状复叶垂直悬挂于风洞中,在风速0~25m/s范围内进行正面和反面迎风测试.发现存在前期稳定、中间过渡和后期稳定3个阶段以及5个临界风速.在前期阶段叶轴随风速弯曲变化剧烈,出现小叶分层飞翼和分层多形状稳定.过渡阶段出现叶轴大幅低频振动和部分小叶小幅高频振动两种不稳定形式.后期出现两层或单一整体稳定,横截面形状分为锥形、楔形和U形.随着风速增大,复叶宽度减小,小叶层数逐步减少,直至出现流线形单一整体.随着雷诺数增大,复叶阻力系数先是快速下降,后又缓慢地趋于常数.复叶Vogel负指数绝对值$\vert \alpha \vert$随小叶数目的增大而增大.反面迎风时$|\alpha|$比正面迎风时大,但随着小叶数目增加两者趋于一致.当复叶旋涡脱落频率与叶轴固有频率接近时,叶轴出现大幅振动.理论分析得到叶轴振动的第二临界风速$V_2/\sqrt{E/\rho}$是$b/l$和$d/l$的函数,其中$E$,$\rho$,$d$和$l$分别为叶轴弹性模量、密度、直径和长度,$b$为变形后的复叶宽度,并由实验数据得到了其变化图.      

Dynamics of a Flexible Slider-Crank Mechanism Driven by a Non-Ideal Source of Energy

The dynamic response of a slider crank mechanism with a flexible connecting rod driven by an electric DC motor is examined. After formulating the governing nonlinear boundary value problem, it is reduced by a one-term truncation to a system of nonlinear ordinary differential equations representing the complete dynamics of the electromechanical system. First, the steady-state behavior is analyzed. It turns out that a constant crank rotational speed is not possible: fluctuations appear which can be limited by an appropriate choice of the system and the engine data. Then, the transient startup or rundown is dealt with by using a digital simulation of the model equations. It exhibits a rich variety of dynamic effects till the stationary speed is reached. Attention is focused on the influence of the flexibility which in both categories of motion yields typical features.     

弹性转子-滑动轴承系统稳定性分析

对一人考虑不平衡力和非线性油膜力的弹性转子 -轴承系统 ,用数值方法研究其稳定性和油膜失稳运动 (自激振动 )特性 ;推导出转子受冲击的运动模型 ,并分析了冲击对系统稳定性的影响规律。     

PIV analysis of in-cylinder flow structures over a range of realistic engine speeds

In-cylinder PIV measurements have been performed in a four-valve single cylinder optical gasoline direct injection engine, motored at speeds of 750, 2,000 and 3,500 rpm. Mean vector flow fields have been produced during the latter half of the intake stroke in the symmetry plane between the valve pairs. The flow fields show the development of the in-cylinder flow structures at 1.6, 2.4 and 3.2° crank angle steps for a time period of approximately 5 ms at each of the three engine speeds, respectively. Tumble ratios have been calculated for the available field of view showing a change in the flow structure between 2,000 and 3,500 rpm. This is believed to be caused by an increase in the flow of air traveling underneath the recirculation vortex at the higher engine speed. This translates the vortex position upwards and further to the right when compared to the lower engine speeds.     

高速涡轮增压器轴承-转子系统不平衡响应及稳定性分析

汽车涡轮增压器广泛采用浮环轴承支承的小型轻质转子系统,以实现100 000~300 000 r/min的工作转速,提高发动机功率和动力性能,并降低燃油消耗和排放. 在此超高速工况下,动压油膜的强非线性作用和转子固有的不平衡效应使该系统呈现出复杂的动力学现象,其中油膜涡动、振荡、跳跃、倍周期分岔和混沌等非线性动力学行为对增压器的健康运转意义重大,因而备受关注. 本文作者从摩擦学动力学耦合的角度出发,基于流体动压轴承润滑理论和有限差分法计算非稳态油膜压力,结合达朗贝尔原理和传递矩阵法建立了转子离散化动力学方程,提出了一种由双油膜浮环支承的涡轮增压器转子系统动力学模型,并从转子轨迹、轴承偏心率、频谱响应、庞加莱映射和分岔特性等方面比较分析,描述了该非线性轴承-转子系统的不平衡效应及油膜失稳特征. 结果表明:转子一般在相对低速下作稳定的单周期不平衡振动,在高转速下其被油膜失稳引起的次同步涡动所抑制,但不平衡量的增加可阻碍转子以拟周期运动通向混沌运动的路径;适当不平衡补偿下,由于内、外油膜间交互的非线性刚度和阻尼作用,在油膜失稳区间之间的中高速区会出现适合增压器健康运转的稳定区间.      

Experimental investigation on the influence of gap vortex streets on fluid-structure interactions in hexagonal bundle geometries

Gap vortex streets characterise many industrial applications involving rod bundle flows, such as heat exchangers and nuclear reactors. These structures, known as gap vortex streets, may excite the structural components of the bundle to resonance, leading to fretting and fatigue. This work aims to measure these coherent structures and the resulting displacement and oscillation frequency of the neighbouring rod, to provide unique data for fluid-structure interaction studies and to develop a general correlation for estimating the coherent structure’s wavelength. A water loop was built to host a hexagonal rod bundle. Fluorinated Ethylene Prophylene (FEP), a refractive index matching (RIM) material, was used to have undisturbed optical access in the area around the central rod. The flow was measured with Laser Doppler Anemometry (LDA) to detect coherent structures, while the vibrations were measured with a high speed camera. A new correlation for estimating the wavelength of the coherent structures is derived with dimensional analysis based on experimental evidence. The correlation is tested on different geometries: rectangular channels with single or half-rods, and two rod bundles, within the pitch-to-diameter ratio (P/D) range 1.02–1.2. Moreover fluctuations in the flow, given by the detected coherent structures, govern the structural response of the rod. The rod is excited to resonance if these fluctuations match twice the natural frequency of the rod.     

Steady Ostwald flow between two differentially rotating co-axial discs

The steady flow of a power-law (Ostwald) fluid between two differentially rotating, parallel, co-axial discs has been considered for both large and small Reynolds number. All edge effects of the discs are neglected, the discs rotate in the same sense and the distance between the two discs is much smaller than their radius. In the large Reynolds number case a similarity solution is sought. It is assumed that the flow consists of boundary layers on the discs, while the core rotates as a rigid body with speed intermediate of those of the discs. The boundary layer is thinner than in the equivalent Newtonian problem, and the decay of the boundary layers is found to be algebraic. This slow decay contrasts with the faster exponential decay in the Newtonian case. For the low Reynolds number problem, the ratio of the disc separation to radius was taken to be much smaller than the Reynolds number. This is, in effect, a lubrication-type problem. The velocity components are expressed as expansions in ascending powers of the Reynolds number. For both the large and small Reynolds number flow, the torque is calculated as a function of the disc speeds, for various values of the power-law index n.     

Design of piezoaeroelastic energy harvesters

We design a piezoaeroelastic energy harvester consisting of a rigid airfoil that is constrained to pitch and plunge and supported by linear and nonlinear torsional and flexural springs with a piezoelectric coupling attached to the plunge degree of freedom. We choose the linear springs to produce the minimum flutter speed and then implement a linear velocity feedback to reduce the flutter speed to any desired value and hence produce limit-cycle oscillations at low wind speeds. Then, we use the center-manifold theorem to derive the normal form of the Hopf bifurcation near the flutter onset, which, in turn, is used to choose the nonlinear spring coefficients that produce supercritical Hopf bifurcations and increase the amplitudes of the ensuing limit cycles and hence the harvested power. For given gains and hence reduced flutter speeds, the harvested power is observed to increase, achieve a maximum, and then decrease as the wind speed increases. Furthermore, the response undergoes a secondary supercritical Hopf bifurcation, resulting in either a quasiperiodic motion or a periodic motion with a large period. As the wind speed is increased further, the response becomes eventually chaotic. These complex responses may result in a reduction in the generated power. To overcome this adverse effect, we propose to adjust the gains to increase the flutter speed and hence push the secondary Hopf bifurcation to higher wind speeds.     

石墨烯裂纹扩展行为研究

基于分子动力学方法对含预制裂纹石墨烯进行扶手椅向拉伸断裂模拟。使用连续介质理论结合分子动力学计算石墨烯能量释放率,确定石墨烯能量释放率GIC为10.25 J/m2;应力强度因子KIC为 3.33MPam^1/2。进一步对影响石墨烯裂纹扩展速率的因素-初始裂纹长度与加载速率进行讨论。结果表明：裂纹初始长度与加载率会在一定程度上影响石墨烯中裂纹扩展速率。裂纹扩展速率会随着初始裂纹长度的增加而降低;但随着初始裂纹长度的增加,裂纹扩展速率对其敏感度降低。裂纹扩展速率会随着加载率的升高而增大。 初始裂纹长度与加载率对裂纹扩展速率的影响有一定的关联性,加载率的升高会降低裂纹扩展速率对初始裂纹长度变化的敏感度。在此基础上确定了石墨烯中裂纹扩展极限速率为8350 m/s。关联性,加载率的升高会降低裂纹扩展速率对初始裂纹长度变化的敏感度。在此基础上确定了石墨烯中裂纹扩展极限速率为8350 m/s。     

Dynamics analysis of 2-DOF complex planar mechanical system with joint clearance and flexible links

Joint clearance and flexible links are two important factors that affect the dynamic behaviors of planar mechanical system. Traditional dynamics studies of planar mechanism rarely take into account both influence of revolute clearance and flexible links, which results in lower accuracy. And many dynamics studies mainly focus on simple mechanism with clearance, the study of complex mechanism with clearance is a few. In order to study dynamic behaviors of two-degree-of-freedom (DOF) complex planar mechanical system more precisely, the dynamic analyses of the mechanical system with joint clearance and flexibility of links are studied in this work. Nonlinear dynamic model of the 2-DOF nine-bar mechanical system with revolute clearance and flexible links is built by Lagrange and finite element method (FEM). Normal and tangential force of the clearance joint is built by the Lankarani–Nikravesh and modified Coulomb’s friction models. The influences of clearance value and driving velocity of the crank on the dynamic behaviors are researched, including motion responses of slider, contact force, driving torques of cranks, penetration depth, shaft center trajectory, Phase diagram, Lyapunov exponents and Poincaré map of clearance joint and slider are both analyzed, respectively. Bifurcation diagrams under different clearance values and different driving velocities of cranks are also investigated. The results show that clearance joint and flexibility of links have a certain impact on dynamic behavior of mechanism, and flexible links can partly decrease dynamic response of the mechanical system with clearance relative to rigid mechanical system with clearance.     

Effect of internal flow on vortex-induced vibration of risers

Although there are many studies dedicated to the problem of vortex-induced vibration (VIV) of marine risers, VIV experiments with internally flowing fluid have not been carried out before. In order to investigate this area, the present experiment with an internally flowing fluid and external current was designed. The riser was towed in the water flume with varying internal flow speeds. The tests in still water and in a current were conducted successfully. Various measurements were obtained including the frequency responses and the time-domain tracing of in-line and cross-flow responses. The experimental results exhibit several valuable features. First, with an increase in internal flow speed, the response amplitude increases while the vibration frequency decreases. Secondly, internally flowing fluid lessens the correlation of the vibration between different sections. In addition, by plotting both in-line strain and cross-flow strain simultaneously, a figure-of-eight for bending strain is also observed, and the trajectories in different cycles are more concordant with the increase of internal flow speed.     

多喷口高效能厚翼的研究

提出了以下高效能翼型的思想：用多喷口小速度切向吹气控制厚翼上的流动分离,使流动接近于理想流状况,以产生大升力,小阻力;因多喷口小速度吹气耗能小,故翼型的有效升阻比可以很大．基于雷诺平均N－S方程进行了数值模拟实验．主要结果表明：对于厚度为0.4的儒氏翼型,在升力系数高达3．5时,有效升阻比可达约50（单喷口吹气约为23）;对于厚度为0．4的"升力体"翼型,在升力系数达2．2时,有效升阻比可达40（喷口吹气约为10）．     

Standard Uncertainty Evaluation for Dynamic Tensile Properties of Auto-Body Steel-Sheets

This paper is concerned with the standard uncertainty of the true stress–true strain curve as the tensile properties of auto-body steel sheets at intermediate strain rates ranged from 1 to 100 s^?1. A procedure to obtain true stress–true strain data is properly designed for the experiment and data acquisition. An analytic model is then established to evaluate the standard uncertainty of the measurand. The measurand in this case is the true stress which is a function of the input quantities: the tensile load; the initial length, the thickness and the width of a specimen; and the deformed length of a specimen. Sources of uncertainties of the input quantities are evaluated for the high speed tensile test with their associated sensitivity coefficients. Uncertainty of the stress data acquired is also considered in the procedure of the fast Fourier transform (FFT) smoothing process used to remove unnecessary signals acquired from experiments. Image analysis using a high speed camera is carried out to measure deformation of the specimen during high speed tensile tests with proper uncertainty evaluation. A combined standard uncertainty is evaluated from the uncertainties of the input quantities as well as the influence factor for the true stress of auto-body steel sheets at intermediate strain rates. Consequently, the true stress–true strain data are obtained with proper standard uncertainty evaluation.     

高速铁路连续梁桥的共振响应与振动控制研究

以液体粘滞阻尼器为振动控制外部装置,主桥采用欧拉伯努利梁,通过中国和谐号动车组CRH380AL、日本新干线Shinkansen700和欧洲高速列车HSLMA8三种不同类型的高速列车对比,模拟分析了高速列车作用下桥梁结构共振响应的影响因素,以及粘滞阻尼器的阻尼系数与安装位置对桥梁结构振动响应的减振效果。研究结果表明,（1）合理有效地布置列车荷载轴距,可使桥梁结构发生基频共振的列车时速在运营时速之外,避免了桥梁结构发生较大振动峰值响应,即桥梁结构的基频共振;（2）随着粘滞阻尼器阻尼系数的增大,桥梁的加速度峰值在列车不同时速下均在减少,对桥梁振动有着不同程度的减振效果;（3）通过合理安置液体粘滞阻尼器,可有效降低高速列车作用下桥梁结构的共振响应;（4）随着粘滞阻尼器与主梁的连接点位置逐渐远离支座,粘滞阻尼器的减振效果逐渐明显;（5）随着粘滞阻尼器与桥台的连接点位置逐渐靠近支座,粘滞阻尼器的减振效果略有提升,但不明显。     

Non-Linear vibrations and chaos in harmonically excited rectangular plates with one-to-one internal resonance

Nonlinear flexural vibrations of a rectangular plate with uniform stretching are studied for the case when it is harmonically excited with forces acting normal to the midplane of the plate. The physical phenomena of interest here arise when the plate has two distinct linear modes of vibration with nearly the same natural frequency. It is shown that, depending on the spatial distribution of the external forces, the plate can undergo harmonic motions either in one of the two individual modes or in a mixed-mode. Stable single-mode and mixed-mode solutions can also coexist over a wide range in the amplitudes and frequency of excitation. For low damping levels, the presence of Hopf bifurcations in the mixed-mode response leads to complicated amplitude-modulated dynamics including period doubling bifurcations, chaos, coexistence of multiple chaotic motions, and crisis, whereby the chaotic attractors suddenly disappear and the plate resumes small amplitude harmonic motions in a single-mode. Numerical results are presented specifically for 1 : 1 resonance in the (1, 2) and (3, 1) plate modes.     

On the use of electrical-resistance foil strain gages for plastic-wave studies in solids

The use of electrical-resistance foil strain gages for the measurement of plastic-wave profiles is investigated. It is shown that the wave speeds and maximum strains are obtained to about the same accuracy as optical techniques. It should be noted that the plastic wave speed and the maximum strain are the two most important parameters which are used to infer dynamic response of a material.     

高速旋转弹头侵彻运动金属薄板的数值模拟

提出了一种利用LS-DYNA程序计算弹头翻转角度曲线的方法。在侵彻过程中,弹头的速度为300 m/s,转速分别为0、3 600和6 370 r/s;金属薄板的速度分别为0、40和80 m/s。其中,弹头直径为7.62 mm,圆形金属薄板的直径为80 mm,厚度为2 mm。材料模型选择了考虑应变、应变率效应和温度效应的Johnson-Cook材料模型。通过数值模拟结果的比较来研究不同弹头转速和金属薄板速度对侵彻过程中弹头最终速度、翻转角度和弹道偏移的影响。     

Dynamics of Slider-Crank Mechanisms with Flexible Supports and Non-Ideal Forcing

Transient and steady state dynamic response of a class of slider-crank mechanisms is investigated. Specifically, the class of mechanisms examined involves rigid members but compliant supporting bearings. Moreover, the mechanisms are subjected to non-ideal forcing. Namely, both the driving and the resisting loads are expressed as a function of the angular coordinate describing the crank rotation. First, an appropriate set of equations of motion is derived by applying Lagrange's equations. These equations are strongly nonlinear due to the large rigid body rotation of the crank and the connecting rod, as well as due to the nonlinearities associated with the bearing action and the form of the driving and the resisting loads. Consequently, the dynamics of the resulting dynamical system is examined by solving the equations of motion numerically. More specifically, transient response is captured by direct integration, while determination of complete branches of steady state response is achieved by applying appropriate numerical methodologies. Initially, mechanisms whose crankshaft is supported by bearings with rolling elements and linear stiffness characteristics are examined. Then, numerical results are presented for rolling element bearings with nonlinear stiffness characteristics. Finally, the study is focused on mechanisms supported by hydrodynamic bearings. In all cases, the attention is focused on investigating the influence of the system parameters on its dynamics. Moreover, models with constant crank angular velocity are first analysed, since they provide valuable insight into some aspects of the system dynamics. Eventually, the emphasis is shifted to the general case of non-ideal forcing, originating from the dependence of the driving and the resisting moments on the crankshaft motion.     

The deformation and fracture of balloons

Inflation of balloons provides a straightforward way of achieving large biaxial deformations. Previous studies have shown that when a balloon bursts, crack propagation occurs at very high speed – much higher than would be expected from the low strain modulus and elastic wave velocity of the rubber. The present paper is concerned with studies of the deformation and fracture of cylindrical balloons. On inflation, the deformations of such a balloon pass through an unstable region but subsequently increase monotonically with pressure. In this relatively high pressure region, the ratio of the longitudinal and circumferential extension ratios is broadly in accord with expectations from high-strain elasticity theory when the ratio of the corresponding stresses is taken into account. On bursting, crack speeds up to around 300 m/s in this region. It is shown that these speeds are in accord with large increase in incremental moduli for the highly-strained rubber. Marked changes in crack tip profile observed at very high crack speeds are consistent with control of the rate of growth by inertia rather than by the viscoelastic properties of the rubber (as is believed to be the case at lower speeds). Consistent with this, various elastomers having different glass transition temperatures show similar crack growth behaviour in the very high speed region.