Effect of geometrical and material imperfections on damping flexural vibrations in plates with attached wedges of power law profile

In the present paper, an efficient method of damping structural vibrations using the acoustic black hole effect is further investigated experimentally. This method is based on some specific properties of flexural wave propagation in tapered plates (wedges) of power-law profile that have to be partially covered by narrow thin strips of absorbing layers. Ideally, if the power-law exponent of the profile is equal or larger than two, the flexural wave never reaches the sharp edge and therefore never reflects back, which constitutes the acoustic black hole effect. It has been previously established theoretically and confirmed experimentally that this method of damping structural vibrations is very efficient even in the presence of edge truncations. The present work describes the results of the experimental studies of the effects of manufacturing intolerances on damping flexural vibrations in wedge-like structures of power-law profile. In particular, the effect of mechanical damage resulting from the use of cutting tools to wedge tips is investigated, including tip curling and early truncation, as well as the placement of absorbing layers on different wedge surfaces. Also, the effects of welded and glued bonding of wedge attachments to basic rectangular plates (strips) are investigated. The results show that, although the above-mentioned geometrical and material imperfections reduce the damping efficiency by varying degrees, the method of damping structural vibrations using the acoustic black hole effect is robust enough and can be used widely without the need of high precision manufacturing.     

Point mobility of a cylindrical plate incorporating a tapered hole of power-law profile

The paper describes the results of experimental measurements of point mobility carried out on circular plates containing tapered holes of quadratic power-law profile with attached damping layers. The obtained results are compared to the developed numerical model, as a means of validation. The profiles of the tapered hole in the plates are designed to replicate near zero reflection of quasi-plane waves from a tapered hole in geometrical acoustics approximation, also known as acoustic black hole effect. The driving point mobility measurements are provided, showing a comparison of the results for a constant thickness circular plate, a constant thickness plate with a layer of damping film applied and a plate with a quadratic power-law profile machined into the center, which is tested with a thin layer of elastic damping material attached. The results indicate a substantial suppression of resonant peaks, agreeing with a numerical model, which is based on the analytical solution available for the vibration of a plate with a central quadratic power-law profile. The paper contains results for the case of free boundary conditions on all edges of the plates, with emphasis placed on the predictions of resonant frequencies and the amplitudes of vibration and loss factor.     

Damping of flexural vibrations in circular plates with tapered central holes

The paper develops a numerical approach to the calculation of mobilities for a circular plate with a tapered central hole of power-law profile. The exact solution of the corresponding flexural wave equation that exists for m=2 has been used in the process of the numerical solution of the corresponding boundary problem. Note that this value of m belongs to the power-law range m≥2 associated with zero reflection of quasi-plane waves from a tapered hole in geometrical acoustics approximation. Two cases of added damping in the central hole area have been considered: a thin absorbing layer and a constrained layer. Cross and point mobilities have been calculated for both these cases. The obtained results for point and cross mobilities show a substantial suppression of resonant peaks (up to 17 dB), in comparison with the cases of a plate with an uncovered hole of the same power-law profile and of a reference circular plate of constant thickness covered or uncovered by a thin absorbing layer. Further theoretical and experimental research is needed to examine applications of the obtained numerical results to more practical situations, e.g. to rectangular plates or other structures with arbitrary locations of tapered holes.     

Mitigation of edgewise vibrations in wind turbine blades by means of roller dampers

Edgewise vibrations in wind turbine blades are lightly damped, and large amplitude vibrations induced by the turbulence may significantly shorten the fatigue life of the blade. This paper investigates the performance of roller dampers for mitigation of edgewise vibrations in rotating wind turbine blades. Normally, the centrifugal acceleration of the rotating blade can reach to a magnitude of 7–8g, which makes it possible to use this kind of damper with a relatively small mass ratio for suppressing edgewise vibrations effectively. The parameters of the damper to be optimized are the mass ratio, the frequency ratio, the coefficient of rolling friction and the position of the damper in the blade. The optimization of these parameters has been carried out on a reduced 2-DOF nonlinear model of the rotating wind turbine blade equipped with a roller damper in terms of a ball or a cylinder, ignoring the coupling with other degrees of freedom of the wind turbine. The edgewise modal loading on the blade has been calculated from a more sophisticated 13-DOF aeroelastic wind turbine model with due consideration to the indicated couplings, the turbulence and the aerodynamic damping. Various turbulence intensities and mean wind speeds have been considered to evaluate the effectiveness of the roller damper in reducing edgewise vibrations when the working conditions of the wind turbine are changed. Further, the optimized roller damper is incorporated into the 13-DOF wind turbine model to verify the application of the decoupled optimization. The results indicate that the proposed damper can effectively improve the structural response of wind turbine blades.     

尾缘凹陷轴流风轮内流特性分析与降噪研究

为减少空调室外机用低压轴流风轮叶片的颤振,改善音质,降低噪音和电机功率。本文设计了尾缘凹陷新型三叶轴流风轮,同时除对前缘局部加厚处理外,对叶片其他区域整体减薄。文中采用Navier-Stokes方程对新风轮和原风轮的内流及气动特性进行了三维数值模拟,同时对两者的流量压力曲线和噪音频谱进行了实验研究。研究表明:对轴流风轮叶片尾缘进行凹陷设计,对前缘以外的区域减薄设计,能够减轻风轮重量,降低电机负荷同时减弱转子尾迹;对叶轮前缘进行局部加厚处理可以减小叶尖颤振,降低叶片旋转频率峰值噪音,从整体上降低噪音,改善音质。     

Visualization of aerodynamic noise source around a rotating fan blade

The purpose of this study is to understand the aerodynamic noise source distribution around a rotating fan blade by measuring the noise signal and velocity field around the blade. The local noise-level distribution over the fan blade is measured by microphone arrays, and the flow field is visualized by smoke and phase-averaged PIV measurement. The noise source distribution is examined by cross-correlation analysis between noise signal and velocity fluctuation. It is found that the noise source is located near the rotating fan blade, especially around leading and trailing edges. The separation and reattachment of flow are observed near the leading edge, and the tip vortices and vortex shedding are found near the trailing edge. The cross-correlation distribution of the noise signal and the radial velocity fluctuation shows large magnitude in the correlated regions, which indicates the noise generation by the formation of vortex structure around the blade.     

叶片锯齿尾缘对降低空调室外机气动噪声影响的试验研究

依据现有的叶片尾迹宽度计算公式,计算了一空调室外机风机叶片的尾迹宽度,然后,以此尾迹宽度为参考基准,设计了两组不同形状和大小的锯齿形叶片尾缘,制作并试验研究了锯齿形尾缘对风机气动噪声的影响规律。结果表明,锯齿尾缘有明显降噪效果,正弦形锯齿较正三角形更好。锯齿尾缘通过降低宽频噪声降噪,而对离散噪声影响很小。采用与叶片尾迹...     

Damping of flexural vibrations in rectangular plates using the acoustic black hole effect

The reduction of flexural vibration in plate structures has been investigated using the recently reported acoustic black hole effect for flexural wave reflection in plates with the local thickness varying according to h(x)=εx^m and m≥2. Since sharp edges of such plates (wedges) are always truncated before x=0, the real reflection coefficients are relatively high, therefore the application of a small amount of damping is required to achieve large reductions in vibration amplitude. This paper presents a numerical model of a plate incorporating an acoustic black hole wedge, with predictions for vibration amplitudes. These are compared to equivalent experimental measurements for a range of applied damping layers. It is concluded that the above-mentioned power-law wedges can be used as effective vibration dampers in plate structures over a wide frequency range of interest.     

缝隙抽吸对液膜撕裂特性影响的试验研究

本文在空气-水膜两相流动试验装置上,利用高速摄影机对空心静叶缝隙抽吸下的静叶出口边液膜撕裂特性进行了试验研究,结果表明:缝隙抽吸可以将静叶表面上的大部分流动液膜去除掉,水份在叶片出口边之前形不成完整的流动液膜,仅表现为溪状流动,静叶出口边的撕裂形式主要为丝线状撕裂,撕裂形成的水滴数目和直径将明显地减小。     

非光滑叶片对轴流风扇气动性能的影响

在低速轴流风扇实验台上,通过对比测量光滑转子叶片、两种表面具有流向沟槽的非光滑转子叶片风扇的气动性能,研究了叶片沟槽面对风扇气动性能的影响.结果表明:(1)在设计状态和小流量状态,采用微槽型非光滑叶片能提高该风扇的流量、降低总压损失和提高风扇总压升;(2)在近失速状态,非光滑叶片使风扇性能下降;(3)非光滑叶片对风扇性能的影响在很大程度上取决于非光滑叶片的沟槽尺寸.     

Large scale correlations for energy injection mechanisms in swirling turbulent flows

We report an experimental study of large scale correlations in the power injected in turbulent swirling flows generated in the gap between two coaxial rotating disks. We measure the pressure fluctuations on the blades of one disk, as well as the pressure drop between the leading and the trailing edges of the rotating blades, i.e. the local drag force. Measurements at different positions on one blade and on two successive blades display a correlation length much larger than the ones usually expected in turbulent flows. The time lag for which the correlation between two points is maximum, strongly depends on the global flow configuration. These results help us to understand the statistical properties of the injected power fluctuations in turbulent swirling flows. Received 2 September 1999     

Exact solutions of equation of transverse vibrations for a bar with a specific cross section variation law

Flexural wave propagation along a bar whose thickness smoothly decreases down to zero within its end piece is considered. The propagation velocity tends to zero as the tapered end of the bar is approached, and the time of wave propagation to the tapered end is infinite. As a consequence, waves propagating along the bar are not reflected from the end. Previous quantitative study of the effect in the WKB approximation shows that, in the case of parabolic tapering, the WKB approximation yields a uniform asymptotics, which is valid (or invalid) for any of the bar’s cross sections. In the case of a bar with parabolic tapering, the equation of flexural vibrations of the bar has exact analytic solutions in the form of power functions. Based on these solutions, a modified WKB approximation is proposed to solve equations for bars with nonparabolic thickness variation laws. The input impedance of a bar with a parabolic tapering is calculated and analyzed.     

Vortex shedding noise of low tip speed,axial flow fans

Noise and performance tests were conducted on three low tip speed, half-stage, axial flow fans to determine the nature of the vortex shedding noise mechanism. Each fan was 356 mm in diameter and had eight equally spaced, variable pitch blades. The noise measurements were made in a free field environment and the fan back pressure and speed were varied during the tests. An acenaphthene coating on the blades was used to determine the regions of laminar and turbulent flow.Vortex shedding can be a significant source of noise when the fan is operated in a lightly loaded condition. Essentially it is due to instabilities in the laminar boundary layer on the suction side of the blade where these instabilities are in the form of Tollmien-Schlichting (T-S) waves. These instabilities interact with the trailing edge of the blade and generate acoustic waves which radiate from the trailing edge and form a feedback loop with the source of the instabilities. Vortex shedding noise can contribute as much as 5 dB in overall noise level and up to 22 dB at higher frequencies (8–14 kHz).Serrations located at the leading edge, at the mid-chord, or near the trailing edge on the suction side were found to reduce the vortex shedding noise significantly. The mid-chord location was found to be the most satisfactory because, as well as eliminating the noise, the serrations provided a 3% improvement in peak efficiency. This improvement occurred because separation of the laminar boundary layer was prevented on the suction side. On the other hand, serrations placed at the other two locations tended to degrade fan performance.     

海上风电叶片智能控制实验系统研制及控制效果分析

随着风力机大型化发展,叶片尾缘襟翼控制技术,作为叶片流场主动控制的一种有效手段,能够有效、快速、灵活地降低叶片载荷,提高风力机,特别是大型风力机的可靠性、经济性,该技术受到国内外的广泛关注.为深入了解叶片襟翼实际作用效果及降载机制,在大量数值仿真计算工作基础上,需进一步开展带有襟翼控制的模型风力机风洞实验工作.本文在相...     

后掠叶片锯齿尾缘宽频噪声实验研究

本文采用线性传声器阵列分别对具有常规尾缘及锯齿形尾缘的后掠叶片的尾缘噪声进行了实验测量;运用CLEAN-SC数据处理方法精确地识别出叶片尾缘噪声的声学参数.并且基于多组实验结果的对比,深入研究了不同的尾缘锯齿长度、周期、几何比例对后掠叶片尾缘噪声降噪效果的影响.实验结果表明:在低湍流度、自由来流情况下,在总声压级降噪方...     

大小叶片贯流风机内流特性分析与实验研究

为降低空调用贯流风机的噪声,改善音质,通过采用直叶片贯流风轮达到斜扭叶片贯流风轮的音质和低噪声特性,从而降低贯流风轮的制造成本,本文设计了大小叶片交错组合的新型非等距贯流风机,并采用滑动网格对其内流特性进行了非定常数值模拟,同时对其气动噪声特性进行了实验研究.大小叶片贯流风机的偏心涡基本稳定在叶轮中心与蜗舌相连的切线上,位于叶轮内圆周附近.风轮非定常运转时,偏心涡的涡核位置在直径为2mm的圆所围成的区域内变化.大小叶片交错组合的贯流风轮改变了叶轮与蜗舌的间距,有效地降低叶片通过频率噪声并改善了音质.采用大小叶片交错组合的贯流风轮能够达到斜扭叶片贯流风机的降噪效果.     

Effect of interactive damping on vibration sensitivities of a scanning near-field optical microscope probe

The effect of interactive damping on the sensitivity of flexural and axial vibration modes of scanning near-field optical microscope (SNOM) with a tapered optical fiber probe has been analyzed. The interaction of the SNOM probe with a sample surface is modeled by a combination of a spring and a dashpot in the flexural direction and a similar combination in the axial direction. An approximate form for the sensitivities of both modes was derived by using the Rayleigh–Ritz method. The results show that the interactive damping will decrease the sensitivities of both flexural and axial vibration modes when the contact stiffness is low. The more the damping effect, the lower the sensitivities are. In addition, when the contact stiffness was low, the flexural sensitivity of the tapered probe slightly increased as the tapered angle decreased. However, the axial sensitivity apparently decreased as the tapered angle decreased. When the contact stiffness became higher, the sensitivities of both flexural and axial vibration modes increased as the tapered angle increased. PACS 68.35.Ja; 07.79.Fc; 61.16.Ch     

Measurements of radiation vibrations of turbomachine blades using an optical-fiber displacement-sensing system

Turbomachine blades are critical equipment in the energy, chemical, aviation, and shipbuilding industries. Turbomachine-blade vibrations can cause high cycle fatigue, which reduces the blade lifetime. Their stable operation is a determining factor of safe and efficient production. In order to monitor and detect the turbomachine-blade vibrations and check whether whole or partial performance is normally operating, we design a reflective intensity-modulated optical-fiber sensing system for radial vibration detection of turbomachine blades and introduce the basic principles of the detection system in detail. We study some key technologies such as the control system of the laser-diode (LD) constant power and an optical-fiber coupling system with the optical-fiber-bundle structure. We analyze the sensor output characteristics and present some numerical simulations. In view of our experimental results, we show that the system elaborated can eliminate the effects caused by light-intensity fluctuations, optical-fiber flexural losses, and changes in the surface reflection coefficient, and can detect the radial vibrations of turbomachine blades in the presence of strong electromagnetic interference and under high temperatures.     

锯齿尾缘降噪翼型优化设计研究

大量研究工作表明旋转风电叶片的主要气动噪声来自叶尖尾缘区域,一直以来都是严重影响居民生活和叶片气动性能发挥的重要因素之一.为此,针对决定叶片重要气动特性单元——二维翼型,采用有别于传统的仿猫头鹰翅膀锯齿尾缘流动控制方法,将锯齿关键尺寸参数融入到风力机翼型设计之中,从而开发仿生锯齿翼型的优化设计方法,获得低噪声与高气动性...     

Velocity field measurements of swirl flow around a forward-swept axial fan using a phase-averaged PTV

The flow characteristics around a rotating axial fan were experimentally investigated using a phase-averaged PTV velocity field measurement technique. The axial fan has 5 forward-swept blades with a radius of 25 mm. Measurements were carried out at 4 axial planes and 4 radial planes perpendicular to the axis of rotation. For the axial plane measurements, one fan blade was divided into 4 different phases in order to analyze the flow structure according to blade phase. For each case, 500 instantaneous velocity fields were measured and ensemble averaged to obtain phase-averaged velocity vector fields and vorticity contours. In addition, measurements were carried out at two planes around the blade surface. Phase averaged velocity fields show periodic variations with respect to the blade phase. The periodic shedding of the tip vortex at the blade tip is also observed. The phase averaged velocity fields measured in the radial planes show periodic variations according to the fan phase and the distance from the fan. These experimental results can be used to validate numerical calculations and to understand the flow characteristics of forward-swept axial fans.