A Novel Wake Oscillator Model for Vortex-Induced Vibrations Prediction of A Cylinder Considering the Influence of Reynolds Number

It is well known that the Reynolds number has a significant effect on the vortex-induced vibrations(VIV) of cylinders. In this paper, a novel in-line(IL) and cross-flow(CF) coupling VIV prediction model for circular cylinders has been proposed, in which the influence of the Reynolds number was comprehensively considered. The Strouhal number linked with the vortex shedding frequency was calculated through a function of the Reynolds number. The coefficient of the mean drag force was fitted as a new piecewise function of the Reynolds number, and its amplification resulted from the CF VIV was also taken into account. The oscillating drag and lift forces were modelled with classical van der Pol wake oscillators and their empirical parameters were determined based on the lock-in boundaries and the peak-amplitude formulas. A new peak-amplitude formula for the IL VIV was developed under the resonance condition with respect to the mass-damping ratio and the Reynolds number. When compared with the results from the experiments and some other prediction models, the present model could give good estimations on the vibration amplitudes and frequencies of the VIV both for elastically-mounted rigid and long flexible cylinders. The present model considering the influence of the Reynolds number could generally provide better results than that neglecting the effect of the Reynolds number.

COMPLEX NUMBER MODE APPROACH FOR REDUCING VIBRATION OF STRUCTURAL FLEXIBLE REDUNDANT ROBOT MANIPULATORS

A complex number mode analysis approach is proposed for vibration reducing of structural flexible redundant manipulators by utilizing self motion. In the proposed approach, the self motion is evaluated to nullify the modal exciting force of flexural motion, and the approach can be freely used when the degree of freedom of flexural motion is much greater than the available degree of reundancy. The availability and effectiveness of the proposed approach are demonstrated through numerical simulation with a four link spatial robotic manipulator possessing an end flexible link.

Analysis of ground vibrations due to underground trains by 2.5D finite/infinite element approach

The 2.5D finite/infinite element approach is adopted to study wave propagation problems caused by underground moving trains. The irregularities of the near field, including the tunnel structure and parts of the soil, are modeled by the finite elements, and the wave propagation properties of the far field extending to infinity are modeled by the infinite elements. One particular feature of the 2.5D approach is that it enables the computation of the three-dimensional response of the half-space, taking into account the load-moving effect, using only a two-dimensional profile. Although the 2.5D finite/infinite element approach shows a great advantage in studying the wave propagation caused by moving trains, attention should be given to the calculation aspects, such as the rules for mesh establishment, in order to avoid producing inaccurate or erroneous results. In this paper, some essential points for consideration in analysis are highlighted, along with techniques to enhance the speed of the calculations. All these observations should prove useful in making the 2.5D finite/infinite element approach an effective one.

Variational Mode Decomposition for Rotating Machinery Condition Monitoring Using Vibration Signals

The failure of rotating machinery applications has major time and cost effects on the industry.Condition monitoring helps to ensure safe operation and also avoids losses.The signal processing method is essential for ensuring both the efficiency and accuracy of the monitoring process.Variational mode decomposition(VMD)is a signal processing method which decomposes a non-stationary signal into sets of variational mode functions(VMFs)adaptively and non-recursively.The VMD method offers improved performance for the condition monitoring of rotating machinery applications.However,determining an accurate number of modes for the VMD method is still considered an open research problem.Therefore,a selection method for determining the number of modes for VMD is proposed by taking advantage of the similarities in concept between the original signal and VMF.Simulated signal and online gearbox vibration signals have been used to validate the performance of the proposed method.The statistical parameters of the signals are extracted from the original signals,VMFs and intrinsic mode functions(IMFs)and have been fed into machine learning algorithms to validate the performance of the VMD method.The results show that the features extracted from VMD are both superior and accurate for the monitoring of rotating machinery.Hence the proposed method offers a new approach for the condition monitoring of rotating machinery applications.

Flow-Induced Stream-Wise Vibration of Circular Cylinders

Results from a series of studies on the stream-wise vibration of a circular cylinder verifying Japan Society of Mechanical Engineers Standard S012-1998, Guideline for Evaluation of Flow-induced Vibration of a Cylindrical Structure in a Pipe, are summarized and discussed in this paper. Experiments were carried out in a water tunnel and in a wind tunnel using a two-dimensional cylinder model elastically supported at both ends of the cylinder and a cantilevered cylinder model with a finite span length that was elastically supported at one end. These cylinder models were allowed to vibrate with one degree of freedom in the stream-wise direction. In addition, we adopted a cantilevered cylinder model that vibrated with two degrees of freedom in both the stream-wise and cross-flow directions under the same vibration conditions as an actual thermocouple well. The value of the Scruton number (structural damping parameter) was changed over a wide range, so as to evaluate the value of the critical Scruton number that suppressed vibration of the cylinder. For the two-dimensional cylinder, two different types of stream-wise excitations appeared in the reduced velocity range of approximately half of the resonance-reduced velocity. For the stream-wise vibration in the first excitation region, due to a symmetric vortex flow, the response amplitudes were sensitive to the Scruton number, while the shedding frequency of alternating vortex flow was locked-in to half of the Strouhal number of vibrating frequency of a cylinder in the second excitation region. In addition, the effects of the aspect ratio of a cantilevered cylinder on the flow-induced vibration characteristics were clarified and compared with the results of a two-dimensional cylinder. When a cantilevered circular cylinder with a finite length vibrates with one degree of freedom in the stream-wise di-rection, it is found that acylinder with a small aspect ratio has a single excitation region, whereas a cylinder with a large aspect ratio has two excitation regions. Furthermore, the vibration mechanism of a symmetric vortex flow was investigated by installing a splitter plate in the wake to prevent shedding of alternating vortices. The vibration amplitude of acylinder with a splitter plate increased surprisingly more than the amplitude of a cylinder without a splitter plate. For a cantilevered cylinder vibrating with two degrees of freedom, the Lissajous figure of vibration of the first excitation region shows the trajectories of elongated elliptical shapes, and in the second excitation region, the Lissajous trajectories draw a figure “8”. The results and information from these experimental studies proved that Standard S012-1998 provides sufficient design methods for suppressing hazardous vibrations of cylinders in liquid flows.

Effect of Thermal Convection on Viscosity Measurement in Vibrational Viscometer

This paper describes the experimental study of viscosity measurement error in the vibrational type viscometer, which measures viscous damping of the oscillating circular plate in a fluid in continuously increasing temperatures. The experiments are carried out to measure non-uniformity of the temperature field in the test cup of the vibrational viscometer in continuously increasing temperatures, while changing the viscosity of the target fluids. Experimental outcomes show that non-uniformity of the temperature grows in the cup and results in viscosity measurement error, when the viscosity of the fluid increases. In order to understand this phenomenon, velocity measurement by particle image velocimetry is conducted in the test cup for fluids of varying viscosity. The results indicate that mixing is enhanced in the low-viscosity fluid by the occurrence of unsteady thermal convection, while weaker convection appears in the high-viscosity fluid.

Effect of Peak Perforation on Flow Past A Conic Cylinder at Re=100:Drag,Lift and Strouhal Number

A flow past a circular-section cylinder with a perforated conic shroud, in which the perforation is located at the peak of the conic disturbance as the shroud installed on the cylinder and uniformly distributed with several circular holes, is numerically simulated at a Reynolds number of 100. Two factors in the perforation are taken into account, i.e. the attack angle relative to the direction of incoming flow and diameter of holes. The effect of such perforation on the drag, lift and vortex-shedding frequency is mainly investigated. Results have shown that variation of the attack angle has a little effect, especially on the drag and vortex-shedding frequency, except in certain cases due to the varied vortex-shedding patterns in the near wake. The increasing hole diameter still exhibits a little effect on the drag and frequency of vortex shedding, but really reduces the lift, in particular at larger wavelength, such as the lift reduction reaching almost 66% 68% after introducing the perforation.

Control of Vortex Shedding at Moderate Reynolds Numbers

The suppression method of vortex shedding from a circular cylinder has been studied experimentally in the Reynolds number range from 300 to 1600. The test is performed in a water channel. The model cylinder is 1 cm in diameter and 38 cm in length. A row of small rods of 0.18 cm in diameter and 1.5 cm in length are perpendicularly connected to the surface of the model cylinder and distributed along the meridian, The distance between the neighboring rods and the angle of attack of the rods can be changed so that the suppression effect on vortex shedding can be adjusted. The results show that vortex shedding can be suppressed effectively if the distance between the neighboring rods is smaller than 3 times and the cylinder diameter and the angle of attack is in the range of 30degreesless than or equal tobeta<90&DEG;.

低温振动非平衡流动数值模拟研究

在高速低密度风洞喷管流动中,振动温度出现冻结,与平动温度和转动温度产生了严重的非平衡现象。相关数值模拟结果中,振动温度完全冻结,而试验结果明显低于计算结果。为合理解释此现象,对数值模拟方法在低温条件下的应用进行分析,基于直接模拟蒙特卡罗(direct simulation Monte Carlo,DSMC)方法中热力学非平衡模拟技术,对振动松弛碰撞数进行修正,给出了修正系数1×10^(-7),建立了低温振动非平衡流动数值模拟方法。利用该方法对低密度风洞M16喷管流场和10 N姿控发动机羽流流场开展数值模拟,得到了与试验结果相符合的振动温度。研究表明:在低温条件下,由于振动松弛碰撞数很大,气体分子很难发生振动松弛碰撞,振动能不能向平动能转化,振动温度完全冻结;通过修正,降低振动松弛碰撞数,增加振动松弛碰撞的概率,可以降低振动冻结温度,使得数值模拟结果与试验结果吻合。

桥梁风工程研究中的雷诺数效应研究进展

桥梁风工程研究中的雷诺数效应问题是伴随着风洞试验方法产生的一个基础性问题,随着桥梁尺寸的增大其雷诺数也逐渐增大,风洞试验雷诺数和实桥梁雷诺数的差别对试验结果的影响越来越受重视。该研究阐述了雷诺数效应问题的由来,回顾了雷诺数效应研究的发展历程,总结了针对圆形断面、矩形断面、主梁断面等不同桥梁结构断面雷诺数效应问题的研究进展,包括结构周围流场、气动力、风致振动随雷诺数的变化,指出了桥梁风工程雷诺数效应研究中尚待解决的问题,为今后该领域的研究提供参考和建议。

高雷诺数下质量比对圆柱涡致振动影响分析

涡致振动分析可为柔性结构抗风研究与工程应用提供重要理论依据。采用基于非平稳雷诺均值Navier-Stokes方程(Unsteady Reynolds-Averaged Navier-Stokes equations,URANS)的数值方法模拟了高雷诺数、高质量比条件下单向自由度圆柱的涡激振动。在改进的剪切应力输运k-ω(Shear Stress Transport,SST k-ω)模型基础上,运用动网格技术实现流固耦合,并结合Newmark-β方法求解结构运动方程。在雷诺数为1.39×104~1.04×105、折算速度为2~12的条件下,模拟分析了圆柱在不同折算速度时的涡激振动响应,得到了不同质量比时圆柱涡激振动的振幅比、振动频率比、升力系数等的变化特点,讨论了圆柱振动响应和旋涡脱落特性,分析总结了高雷诺数下不同质量比的圆柱涡激振动特性规律。结果表明:数值结果与已有试验的吻合程度较好;雷诺数和质量比对圆柱的涡激振动影响效果明显,对进一步分析圆柱涡致振动具有参考意义。

转子槽数对无刷交流励磁机的空载噪声影响

为减少无刷交流励磁机的电磁噪声,提出一种无刷交流励磁机径向电磁力和交变弯曲力矩的解析模型。根据磁导法,得到空载气隙磁场的解析表达式,进而推导出径向电磁力和交变弯曲力矩的表达式,得到励磁机电磁噪声的主要阶次和转子槽数的关系。通过增加转子槽数可以有效削弱主极磁场和一阶齿谐波引起的噪声,多物理耦合有限元仿真和傅里叶方法验证了结果。

振动环境下单相喷雾冷却试验研究

车载电子设备的热管理系统均涉及振动条件,而有效的振动控制能够改善热环境中流体的对流传热,但目前振动工况对单相喷雾冷却传热性能的影响机制尚未明晰。为研究各种振动工况下单相喷雾冷却的传热性能,本文构建了一个闭环振动表面喷雾冷却系统,结合试验研究,探讨了振动雷诺数(Re_(v))、量纲为一加速度数(A_(c))、振幅及频率对单相喷雾冷却传热性能的影响机制。研究表明:Re_(v)和振幅的增加会导致传热系数和传热增强因子被抑制,当加热功率为200W、Re_(v)为4947时,喷雾冷却被抑制了37%;相对地,高A_(c)和频率可以改善喷雾冷却的传热,在加热功率为200 W、A_(c)为3.6时,喷雾冷却传热性能增强了17%。研究结果可供基于振动工况下的喷雾冷却热管理系统优化参考。

人行桥人致横向振动及横向锁定分析

为了研究人群荷载下人行桥的人致振动特性,采用Duhamel积分求解了行人上下桥过程中简支梁桥的人致横向振动理论解,分析了横向锁定条件和临界人数。结果表明:与原地踏步相比,行人上下桥过程中,桥梁横向振动响应增大;频率比为1.0时,跨中横向加速度最大,产生横向锁定的同步临界人数最少;随着桥上总人数增加,跨中横向加速度最大值线性增加,但超过临界人数后由于同步概率的增加使增长幅度提高;在临界人数附近桥梁发生横向锁定,行人舒适性接近“较差”等级下限值。

高雷诺数下交叉圆柱涡激振动的数值分析

基于能量捕获应用场景,利用数值仿真的方法研究了高雷诺数下交叉圆柱的涡激振动(Vortex Induced Vibration,VIV)现象。首先建立了三维VIV数值模型,通过与已有文献试验数据对比验证了该数值模型的准确性。进而在该模型的基础上,对交叉圆柱的VIV进行了仿真研究,并与普通圆柱做了对比分析。研究发现,受竖圆柱的影响,交叉圆柱在中低流速的涡强度有所减小,使低流速下的柱体振幅减小,启动流速变大;在中流速范围,涡脱落位置的改变,使得振幅增加;在高流速范围涡强度和涡脱落位置变化并不明显,振幅也与普通圆柱相近。

随机数编码融合长脉冲级联调制提升Φ-OTDR定位性能

为解决随机数编码造成相位敏感光时域反射系统振动定位差的问题,设计了一种随机数编码融合长脉冲的级联调制方法,分别利用声光调制器和半导体光放大器实现随机数编码和长脉冲的融合调制,提升消光比。使用自相关的随机数编码生成脉冲序列,设计数字正交解调算法获得拍频信号幅值与相位信息。实验结果表明,在4km长度传感光纤上,脉冲宽度为40ns时,方案可将消光比提升13.7dB;当振动信号频率为200Hz时,振动信号定位均方根误差为2.1m,最大绝对误差为3.8m,提升了振动信号定位精度,为编码相位敏感光时域反射系统定位性能提升提供了新方法。

叶片数对离心泵内流诱导振动噪声的影响

基于虚拟仪器数据采集系统和泵产品测试系统在离心泵闭式试验台上建立了泵空化诱导振动噪声试验测试系统,实现了泵能量性能参数和流动诱导振动噪声信号的同步采集.以一台比转数为93的5叶片单级单吸离心泵作为研究对象,在保证泵体和叶轮其他几何参数不变的情况下,将叶轮叶片数分别设计为4,6和7片.测量了不同叶片数下模型泵在全流量范围内的能量性能、振动强度和噪声信号,并对振动和噪声信号数据进行了处理.试验结果表明:模型泵内部流动诱导的振动对泵体的影响最大,5叶片时模型泵的振动强度最高;随着流量的增大各测点振动强度大致都呈现先基本不变后显著上升的趋势;泵的噪声信号主要集中在0～2 000 Hz的范围内;在小于设计流量时,随着叶片数的增大,轴频的峰值逐步增大.研究结果对于建立低振动低噪声离心泵水力设计方法具有重要的参考意义和指导作用.

新型少齿差行星齿轮装置振动特性分析与实验研究

对某少齿差行星齿轮装置进行结构改进设计,运用动力接触有限元法求得包括啮合刚度激励、误差激励、啮合冲击激励的轮齿啮合内部动态激励。通过建立有限元模态动力分析模型,计算齿轮装置固有频率及振型,综合考虑内外部激励情况下,研究该齿轮装置的结构动态响应及噪声。利用振动测试设备对该齿轮装置进行振动噪声测试。结果表明:分析预测值与实验结果相符。

叶片数对离心泵空化诱导振动噪声的影响

为研究叶片数对离心泵空化诱导振动噪声的影响,以一台5叶片单级端吸离心泵为研究对象,保持泵体和叶轮其他几何参数不变,将叶片数分别改为4、6和7.基于虚拟仪器数据采集系统和泵产品测试系统在离心泵闭式实验台上测量了不同叶片数模型泵在不同净正吸入水头(NPSH)时的振动、噪声和出口压力脉动信号,并对信号进行处理和分析.试验结果表明:模型泵空化诱导的振动对泵体的影响最大,综合比较各测点的振动强度,4叶片模型泵的振动最剧烈,5叶片模型泵的振动相对最小.在不同空化状态下,模型泵噪声信号随叶片数的增加叶频峰值基本不变,但轴频峰值变化规律复杂,在空化严重时,6叶片模型泵的轴频峰值最大.随着空化程度的加剧,各叶片数模型泵的轴频峰值呈下降趋势.

钢桁拱桥吊杆风致振动研究

以某钢桁拱桥的吊杆的风致振动为工程背景,研究了拱桥常用的吊杆类型,分析了雷诺数Re、斯托劳哈尔数Sr、吊杆结构振动因子λl及吊杆的结构动力特性对钢桁拱桥吊杆振动的影响程度,给出了用吊杆结构振动因子λl和雷诺数Re相结合的判别吊杆是否发生涡激共振的方法,所得的结论与实际桥梁涡激共振现象一致.提出的方法可用于对钢桁拱桥吊杆的风致振动的判别.