金字塔点阵圆筒结构轴向弹性波传递特性研究

点阵结构的周期和多孔特征使其相较于普通的实体结构具有轻质、耐冲击、减振降噪和高比强度等优势, 在航空航天和船舰等领域具有十分广泛的应用前景. 本文以金字塔点阵圆筒结构为对象, 根据单胞的宽、高和阵列的个数等独立的结构参数进行参数化建模, 并利用商用有限元软件计算了点阵圆筒结构轴向振动特性. 通过无量纲手段针对归一化频率和相对带宽, 系统研究了单胞的宽、高以及杆件直径等关键参数对减振效果的影响. 根据参数分析的结果, 设计了一种具有低频宽带减振性能的金字塔点阵圆筒结构. 使用增材制造技术制作了样件, 并进行了振动测试实验, 结果表明, 实验和有限元模拟的结果基本一致, 在500~1500 Hz范围内, 结构表现出明显的减振效果, 平均的衰减强度达到了50 dB左右.      

旋转圆柱绕流流场特性分析

对雷诺数Re = 20000 ~ 90000、相对转速ɑ = 0 ~ 0.72的旋转圆柱后方流场进行了实验测量, 分析了旋转圆柱后方不同剖面处的速度分布规律和湍流度分布规律. 采用LES方法对旋转圆柱绕流问题进行了数值模拟, 分析旋转圆柱周围流场特性和自由剪切层变化规律, 最后通过理论模型对流场变化进行分析, 得出如下结论: 当圆柱逆时针旋转时, 同一雷诺数下随着相对转速的增加, 旋转圆柱尾迹区域下方速度突变处的位置随着相对转速的增加而上移, 而上方速度突变处的位置不变, 雷诺数的增加使旋转圆柱尾迹区域下方速度突变处位置有小幅度的下移. 通过数值模拟发现, 圆柱旋转之后, 圆柱后方下侧涡的位置明显上移, 且幅度较大. 下方的自由剪切层有明显的上移, 上方的自由剪切层位置变化较小. 最后通过理论分析发现, 圆柱后侧下方涡位置的上移对圆柱升力影响十分显著, 在高雷诺数、低相对转速的条件下, 旋转圆柱后侧下方涡位置的改变对旋转圆柱的升力、尾流区自由剪切层的变化起到了重要的影响.      

VIV suppression of a two-degree-of-freedom circular cylinder and drag reduction of a fixed circular cylinder by the use of helical grooves

Experimental investigations have been carried out to examine the effects of triple-starting helical grooves on the drag of fixed circular cylinders and the vortex-induced vibration of elastically supported cylinders. For the elastically supported cylinder, the Reynolds number varied from 1.3×10⁴ to 4.6×10⁴, whilst for the fixed cylinder from 3.1×10⁴ to 3.75×10⁵. A comparative approach which allows direct comparisons of the results was adopted where two cylinders of identical dimensions and physical properties with or without helical surface grooves were tested in exactly same experimental set-ups. In the elastically supported cylinder tests, the cylinders were attached to a vertically cantilevered supporting rod and towed in a towing tank. Both the in-line and cross-flow vibrations were permitted. In the fixed cylinder tests, the cylinders were supported on rigid vertical struts and towed horizontally in the same towing tank. It is found that for the case investigated the helical grooves were effective in suppressing the vortex-induced cross-flow vibration amplitudes with the peak amplitude reduced by 64%. Drag reductions of up to 25% were also achieved in the sub-critical Reynolds number range tested in the study for the fixed cylinders.     

A modified LuGre friction model for an accurate prediction of friction force in the pure sliding regime

We propose a new friction model based on the well known LuGre friction model that can accurately describe the nature of friction force in the gross sliding regime. The modification is based on the responses observed from a single degree-of-freedom friction-induced vibration system. Numerical analysis shows that the friction curve in the gross sliding regime can only show counter clockwise hysteretic loops without violating other essential features. We then develop a new friction model by modifying the LuGre friction model that can describe both clockwise as well as counter clockwise hysteretic loops in the pure sliding domain.     

Wall proximity effects on the effectiveness of upstream control rod

Two dimensional flow over a circular cylinder with an upstream control rod of same diameter is simulated in unbound condition and in wall bounded conditions. The cylinders are placed at various heights from the wall and the inter-distance between cylinders is also varied. The control rod is subjected to different rotation rates. It is found that, in unbound condition, rotating the control rod decreases the critical pitch length (S/D_cr) and increases the drag and Strouhal number of the main cylinder. In presence of plane wall, the shielding provided by the separated shear layers from the control rod in cavity regime is deteriorated due to deflection of shear layers which results in higher drag and large fluctuation of lift coefficient. However, in wake impingement regime, the binary vortices from the control rod are weakened due to diffusion of vorticity and hence, the main cylinder experiences a lower drag and small lift fluctuations than that of unbound condition. The critical height of vortex suppression (H/D_cr) is higher in cavity regime than that of wake impingement regime due to the single extended-bluff body like configuration. The rotation of control rod energizes the wall boundary layer and increases the critical height of vortex suppression. Increasing the rotational rate of control rod decreases the drag force and reduces the amplitude of lift fluctuation. Analysis of the wall shear stress distribution reveals that it suffers a sudden drop at moderate height where the normal Karman vortex shedding changes to irregular shedding consisting of single row of negative vortices. Modal structures obtained from dynamic mode decomposition (DMD) reveal that the flow structures behind the main cylinder are suppressed due to wall and the flow is dominated by the wake of control rod.     

Control of vortex breakdown in a closed cylinder with a rotating lid

The flow within a closed cylinder with a rotating lid is considered as a prototype for fundamental studies of vortex breakdown. Numerical simulations for various parameter values have been carried out to reproduce the known effect of a thin rotating rod positioned along the center axis as well as analyze the influence of local vorticity sources. As expected, the results show that the breakdown bubbles in the steady axisymmetric flow can be affected dramatically, i.e., fully suppressed or significantly enhanced, by rotating the rod. The main contribution of this article is to show that the observed behavior can be explained by the vorticity generated by the rod locally near the rotating lid and near the fixed lid, as analogous behavior is caused by the introduction of local vorticity sources in the flow without a rod. Moreover, we describe the influence on the breakdown bubbles of the vorticity sources by an analytical model. In addition to improving our understanding, this finding should also open the door to other types of flow control devices capable of generating localized vorticity.     

变速转子准稳态主动平衡系统的增益调度控制

在现代高速旋转机械中,不平衡引起的振动是机器性能降低甚至损坏的重要原因。对变速转子准稳态主动平衡系统,可按一定时间步长把变转速离散成有限个转动角速度。针对每个离散角速度,依据影响系数法,采用包含残余振动值和校正不平衡的广义线性二次型目标性能函数推导出控制律。通过每个离散转速下影响系数的估计和增益矩阵的计算,形成对应于各离散转速的增益表,从而实现变速转子准稳态主动平衡系统的增益调度控制。数值模拟验证了增益调度控制能很好地抑制不平衡振动。与传统的加权二乘法(WLS)相比,数值模拟显示,该法对校正不平衡的惩罚可提高系统的稳定性,也可增强控制的鲁棒性。     

圆柱附加旋转整流罩表面压力分布及气动力特性

对圆柱附加固定整流罩的已有研究表明,它在降低升阻力和抑制涡激振动方面有优良的效果。但固定整流罩具有方向敏感性,当来流方向改变后效果会受到显著影响,甚至起到增加升阻力和加剧涡激振动的反作用。本文给圆柱附加了圆弧直径为40mm,形状夹角α分别为30°、45°、60°、75°和90°五种尺寸的旋转整流罩,并进行了风洞实验。其中整流罩可以自由地围绕圆柱轴线旋转。实验结果表明:旋转整流罩在流体力产生的力矩作用下,旋转至一个偏离尾流中心线固定角度的动态平衡位置,而平衡位置偏转角δ随着形状夹角α的增大而增大。附加旋转整流罩后,相对单圆柱能够提高尾迹区域压力,并能使时均阻力和脉动升力分别在α=30°和α=75°时获得最大43.5%和67.0%的降低。此外,对于小α(α≤60°)情况,漩涡脱落频率明显高于单圆柱情况,而对于大α(α≥75°)情况,则与单圆柱情况相接近。所有旋转整流罩升力主频的幅值较之单圆柱有了很大程度的降低,可见旋转整流罩在抑制漩涡脱落方面有很好的效果。     

The strongest rotating rod

By using the Rayleigh quotient, we present the variational formulation for the strongest rotating rod stable against buckling. This variational formulation is converted to fifth-order singular non-linear boundary value problem. The optimal shape and the critical rotating speed are determined with special numerical-analytical integration procedure. We found the explicit linear relation between the volume and the squared critical speed. Although, in general, the linear stability problem for the optimal rotating rod does not have purely discrete spectra, we show that in the present case, the critical speed is determined with lowest eigenvalue. This fact verifies our optimization strategy based on a linear spectral problem.     

Flow pattern transition in liquid-liquid flows with a transverse cylinder

The effect of a cylindrical bluff body on the interface characteristics of stratified two-phase, oil-water, pipe flows is experimentally investigated with high speed Particle Image Velocimetry (PIV). The motivation was to study the feasibility of flow pattern map actuation by using a transverse cylinder immersed in water in the stratified pattern, and particularly the transition from separated to dispersed flows. The cylinder has a diameter of 5 mm and is located at 6.75 mm from the bottom of the pipe in a 37 mm ID acrylic test section. Velocity profiles were obtained in the middle plane of the pipe. For reference, single phase flows were also investigated for Reynolds numbers from 1550 to 3488. It was found that the flow behind the cylinder was similar to the two dimensional cases, while the presence of the lower pipe wall diverted the vorticity layers towards the top. In two-phase flows, the Froude number (from 1.4 to 1.8) and the depth of the cylinder submergence below the interface affected the generation of waves. For high Froude numbers and low depths of submergence the counter rotating von Karman vortices generated by the cylinder interacted with the interface. In this case, the vorticity clusters from the top of the cylinder were seen to attach at the wave crests. At high depths of submergence, a jet like flow appeared between the top of the cylinder and the interface. High speed imaging revealed that the presence of the cylinder reduced to lower mixture velocities the transition from separated to dual continuous flows where drops of one phase appear into the other.     

浮筏多通道协调振动主动控制实验研究

在浮筏上引入主动吸振器可以有效地改善其低频减振性能.建立了带有4个主动吸振器的浮筏振动主动控制实验系统;以提高系统低频减振性能为目标,研究了4个主动吸振器的协调控制问题.首先,通过实验建模获得系统的时域脉冲序列模型;在此基础上设计了基于多通道滤波x-最小均方(Least Mean Square,LMS)算法的自适应前馈控制律;最后,在实验系统上进行了4个目标点的多通道协调主动吸振控制实验,在15～25Hz频段内基本上都取得了15dB以上的主动减振效果.实验结果证明了多吸振器协调主动控制对提高浮筏低频减振能力的有效性.     

无伞末敏弹气动特性风洞实验研究

为实现末敏弹的无伞稳态扫描运动,设计了一种短圆柱轴向非对称尾翼子弹气动外形。设计了模型和实验装置,进行了大迎角低速风洞实验。获得了模型在有/无减旋翼、固定和自由旋转条件下的气动力数据,测量了模型在气动力作用下的转速。实验结果表明:设计的尾翼能够为模型提供较大的阻力,旋转时模型的阻力系数有所减小;在有减旋翼的情况下,模型能够保持较低的稳定转速;模型的静稳定性较差;模型旋转时稳定性有较大的改善。需要进一步改进尾翼,提高静稳定性,为扫描角的稳定提供保证。实验结果可为无伞末敏弹的设计和改进提供参考。     

Numerical simulation of two-degree-of-freedom vortex-induced vibration of a circular cylinder close to a plane boundary

Two-degree-of-freedom vortex-induced vibrations (VIV) of a circular cylinder close to a plane boundary are investigated numerically. The Reynolds-Averaged Navier-Stokes (RANS) equations are solved using the Arbitrary Lagrangian Eulerian (ALE) scheme with a k-ω turbulence model closure. The numerical model is validated against experimental data of VIV of a cylinder in uniform flow and VIV of a cylinder close to a plane boundary at low mass ratios. The numerical results of the vibration mode, vibration amplitude and frequency agree well with the experimental data. VIV of a circular cylinder close to a plane boundary is simulated with a mass ratio of 2.6 and gap ratios of e/D=0.002 and 0.3 (gap ratio is defined as the ratio of gap between the cylinder and the bed (e) to cylinder diameter (D)). Simulations are carried out for reduced velocities ranging from 1 to 15 and Reynolds numbers ranging from 1000 to 15 000. It is found that vortex-induced vibrations occur even if the initial gap ratio is as small as e/D=0.002, although reported research indicated that vortex shedding behind a fixed circular cylinder is suppressed at small gap ratios (e/D<0.3 or 0.2). It was also found that vibration amplitudes are dependant on the bouncing back coefficient when the cylinder hits the plane boundary. Three vortex shedding modes are identified according to the numerical results: (i) single-vortex mode where the vortices are only shed from the top of the cylinder; (ii) vortex-shedding-after-bounce-back mode; (iii) vortex-shedding-before-bounce-back mode. It was found that the vortex shedding mode depends on the reduced velocity.     

Natural transverse vibrations of a rotating rod

We study the natural transverse vibration frequencies and modes of a rod rotating about an axis fixed at an end of the rod. The cases of low, moderately high, and asymptotically high angular velocities are considered. The case of a homogeneous rod with clamped left and free right end is considered in detail. A new constructive algorithm based on the notion of “sagittary function” is used to find the dependences of the natural frequencies and mode shapes on the angular velocity for lower vibration modes. We establish evolution to the model corresponding to vibrations of a rapidly rotating thread subjected to the centrifugal inertial forces. It is shown that the natural frequencies grow practically linearly with increasing angular rotation velocity. The results obtained can be of interest in technical applications, e.g., when studying vibrations of sensor elements in high-precision instruments or of rapidly rotating elongated mechanism elements (turbine or propeller blades, etc).     

旋转圆板振动的热力控制

本文研究发现,热力对旋转圆盘动力特性有明显影响.可将此特征应用于旋转圆盘振动的热力自适应控制.仿真和试验均表明,这种控制方法对一些结构振动控制将是一种有效的方法.     

The integration of advanced active and passive structural noise control methods

This paper reports on an investigation into the feasibility of using active and passive means of vibration control in aerospace structures. In particular, attention is focused on controlling vibration transmission through light weight satellite structures at medium frequencies. The initial structure under test here is a 4.5-meter long satellite boom consisting of 10 identical bays with equilateral triangular cross sections. This structure is typical of those that might be used in space telescopes, space stations or synthetic aperture radar systems. Such a structure is typically used to support sensitive instruments in precise alignments spaced tens of metres apart. While a great deal of work has been done on this problem at low frequencies, relatively little has been achieved to date at medium frequencies (here taken to be between 150 Hz and 250 Hz). Nonetheless, this is of importance to new space missions. Using the techniques described here, an overall reduction in vibration transmission of 31.0 dB is achieved in an essentially undamped structure using passive means alone. The amounts of attenuation achievable for active control with one, two and three actuators are found to be 15.1 dB, 26.1 dB and 33.5 dB, respectively. With the combined passive control (using 10% geometric deviations) and active control (using three actuators) an overall reduction of 49.5 dB is achievable in practice.     

介电弹性作动器主动隔振模糊控制试验研究

采用模糊控制策略,开展介电弹性作动器的主动隔振性能试验研究。基于介电弹性材料的Maxwell应力模型建立了作动器的力电耦合模型,分析了作动器的非线性特性;针对隔振系统设计了Mamdani型模糊控制器,建立了控制电压信号与振动响应之间的关系;在此基础上,开展了介电弹性作动器主动隔振试验研究,并与加速度反馈控制进行了对比。试验结果表明,在相同驱动电压的情况下,基于模糊控制策略的主动隔振性能要优于加速度反馈控制,且能够显著降低由于非线性驱动力导致的倍频响应幅值,有助于提高隔振系统的稳定性。     

折叠式柔性结构振动主动控制的研究

建立了具有刚性运动基的折叠式柔性结构振动主动控制实验系统，采用带实时预测误差修正的预测控制算法进行了这类结构振动主动控制的实验研究。实验结果表明，在实验建模的基础上设计预测控制器对折叠式柔性结构进行振动主动控制，能够有效地抑制柔性结构的振动和提高刚性基姿态定向精度。     

Control of vortex breakdown in a closed cylinder with a small rotating rod

Effective control of vortex breakdown in a cylinder with a rotating lid was achieved with small rotating rods positioned on the stationary lid. After validation with accurate measurements using a novel stereoscopic particle image velocimetry (SPIV) technique, analysis of numerical simulations using a high-order spectral element method has been undertaken. The effect of a finite length rod creates additional source terms of vorticity as the rod rotates. These additional source terms and their spatial locations influence the occurrence of the vortex breakdown.     

SIMULATION OF TRANSVERSE VIBRATION OF SLENDER ROD STRING IN THE VERTICAL CYLINDER UNDER BUCKLING DISPLACEMENT EXCITATION1)

对于铅直圆筒内受交变拉压轴向载荷作用的细长杆柱,当杆柱底端所受到的轴向压力大于杆柱屈曲的临界载荷时,细长杆柱在圆筒内将产生螺旋屈曲,杆柱的屈曲变形将激励杆柱在圆筒内产生横向振动。以细长杆在圆筒内的瞬时屈曲构型作为杆柱横向振动的位移激励,建立了屈曲位移激励下的细长杆在圆筒内横向振动与杆管碰撞规律的仿真模型。采用有限差分法对井深进行离散,Newmark 法对时间进行离散,以恢复系数法建立了细长杆和圆筒的碰撞条件,对细长杆在圆筒内的横向振动数学模型进行了数值仿真。仿真结果表明,细长杆和圆筒内壁的碰撞现象主要发生在细长杆底端受压发生屈曲后,且几乎沿整个圆筒都有发生,从圆筒顶部至底部的碰撞力峰值逐渐增大;而在杆柱底端受拉时碰撞现象很少,碰撞力也较小。