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...     

正弦锯齿尾缘对轴流风机尾迹及气动性能的影响

轴流风机尾缘涡脱落是产生噪声的重要因素,为改善尾迹流动来降低风机噪声,通过在轴流风机尾缘添加一种正弦形锯齿结构,采用定常及非定常数值模拟的手段结合实验验证的方式,分析其对轴流风机尾迹和气动性能的影响。研究结果显示,正弦锯齿结构削弱了叶片尾缘做功能力,使得风机全压降低,但提升了中小流量工况下风机效率;并能减弱叶片中部以下位置的尾迹强度,且对尾迹的抑制作用从叶片部底到中部逐渐减弱;以增加转速的方式补偿锯齿结构引起的压损,对原型风机和提升转速后的尾缘锯齿结构风机在设计流量点进行噪声数值预测分析,结果显示低频段噪声比原型机有明显改善,表明这种尾缘正弦形锯齿结构一定程度上是一种抑制轴流风机低频噪声的有效途径。     

基于仿生学结构的翼型降噪实验研究

本文运用平面传声器阵列研究了翼型常规尾缘、锯齿形尾缘及波浪形前缘的气动噪声,并在全消声室环境下实验研究了仿生学前缘、尾缘的降噪效果。设计的传声器阵列是同一平面上的单支螺旋线结构,用于二维平面声源定位。实验结果表明:在低湍流度、自由来流情况下,尾缘噪声超过前缘噪声,是翼型噪声的最主要来源,锯齿形尾缘能够有效地降低翼型的尾缘噪声;在较大的宽频范围内,锯齿形尾缘均起到很好的降噪效果,尾缘最大降噪量可以达到10 dB。而波浪形前缘的波长越短,对翼型尾缘的降噪量越明显;波浪形前缘对尾缘噪声的降噪效果在低频时较为明显,而在高频情况下,可以忽略不计。对于湍流-翼型干涉噪声的研究表明:湍流-翼型干涉噪声主要发生于翼型前缘,其声压级远大于尾缘;且锯齿尾缘几乎对湍流-翼型前缘干涉噪声的降噪没有贡献;波浪形前缘可以显著地降低湍流-翼型干涉噪声的声压级,达到良好的降噪效果。     

齿形尾缘轴流叶轮内流数值分析与实验研究

本文采用Navier-Stokes方程和K-ε两方程模型对空调用轴流风轮叶片尾缘出口加锯齿和不加锯齿的内流特性进行了三维数值模拟,同时对两者的风量噪音特性进行了实验研究.结果表明,轴流风轮尾缘进行锯齿化处理对流量压力特性影响很小,能减弱轴流风轮叶片下游的尾迹,并在一定程度上能够降低噪音和改善音质.     

静子尾缘喷气后尾迹与动叶干涉噪声研究

尾缘喷气技术已经广泛地用于航空发动机和多级压缩机等领域,用以降低动静叶片间的相互干涉作用以提高透平机械的气动性能,并降低动静干涉噪声.本文对尾缘喷气用于低压轴流风机进行了详细的研究,对轴流风机的上游静叶实施尾缘喷气,通过实验测量,尾缘喷气使静叶尾迹达到无动量亏损尾迹状态能够降低风机噪声,文章还提出了基于CFD数值模拟的尾迹与动叶相互干涉的噪声预测模型,预测结果和实验结果比较接近.     

离心压气机叶排间涡脱落及气流脉动行为研究

对某一带叶片扩压器跨声速离心压气机内部三维非定常流场进行了数值模拟,依据计算结果描述了叶轮叶片尾缘尾迹涡脱落行为,给出了叶轮尾缘脉动压力频谱特性图和叶轮出口位置脉动速度动能分布图。研究结果表明,主叶片与分流叶片尾缘尾迹涡脱落过程交替进行;转静干涉和转子叶片尾缘的涡脱落行为是造成叶轮尾缘气流脉动行为的主要原因,随着时间的...     

轴流压气机叶片尾迹沿轴向的衰减

本文参考Prandtl关于不可压自由湍流的混合长度理论,研究了轴流压气机叶片尾迹沿轴向的变化规律。得到了尾迹区内的速度分布,尾迹宽度与尾迹区内最大速度亏损值之间的关系式,以及最大速度亏损值沿轴向的衰减公式。理论分析结果与实测结果的比较表明,本文所提供的分析结果可以用来初步预计轴流压气机叶片尾迹沿轴向的衰减程度。     

尾缘结构变化对空调外机轴流风叶声场的影响*

为探索空调外机轴流风叶尾缘结构变化引起的气动声学变化,参照国标噪声测量方法,搭建传声器阵列,测量不同尾缘结构风叶在不同转速下的声压级信息;采用波束形成技术,探究不同尾缘结构风叶在不同转速下的声源位置分布规律。结果表明：与原风叶对比,尾缘凹陷结构风叶、尾缘微孔结构风叶、尾缘锯齿结构风叶均能有效降低气动噪声,其中尾缘凹陷结构风叶可降低噪声1.93 dB-2.78 dB;原风叶、尾缘微孔结构风叶、尾缘锯齿结构风叶声源位置随频段的增加逐渐远离旋转中心,其中在频段Ⅰ四种风叶声源位置都位于轮毂和叶根附近,在频段Ⅱ、Ⅲ、Ⅳ尾缘凹陷结构风叶声源位置分布在尾缘凹陷结构区域附近。为优化风叶气动声学性能提供试验参考。     

对转超音压气机低压转子尾迹对高压转子的作用研究

本文采用数值方法针对对转超音压气机转子叶片排间的相互干涉效应进行了研究,分析了超音转子在两级转子叶片排间具有三种不同轴向间距情况下的性能和前排转子叶片尾迹影响下的超音转子非定常特征。研究结果表明,轴向间距的增大使得尾迹在较长的间距内经过了较为充分的掺混,超音转子最大压比和最高效率随轴向间距的变大而增大;前排叶片尾迹的影响范围从超音转子叶片压力面直至吸力面,横贯整个叶片通道,较高叶展处的尾迹强于较低叶展处;横贯超音转子通道内的低压动叶尾迹先后与斜激波和正激波相互作用,尾迹区内熵进一步增大,并且由于尾迹区的传播速度由主流速度决定,尾迹在通道内继续向下游传播且传播显著延迟于通道外。     

涡轮叶栅尾迹对泄漏流影响的试验研究

本文利用运动圆柱排模拟上游叶片尾迹,在平面涡轮叶栅上研究了上游尾迹与叶栅通道内泄漏流的相互作用,初步讨论了尾迹与泄漏流相互作用的机理,研究发现利用这种相互作用可以控制泄漏涡的强度、减小泄漏损失,进而可实现提高涡轮效率的目的。     

An experimental study of airfoil instability tonal noise with trailing edge serrations

This paper presents an experimental study of the effect of trailing edge serrations on airfoil instability noise. Detailed aeroacoustic measurements are presented of the noise radiated by an NACA-0012 airfoil with trailing edge serrations in a low to moderate speed flow under acoustical free field conditions. The existence of a separated boundary layer near the trailing edge of the airfoil at an angle of attack of 4.2 degree has been experimentally identified by a surface mounted hot-film arrays technique. Hot-wire results have shown that the saw-tooth surface can trigger a bypass transition and prevent the boundary layer from becoming separated. Without the separated boundary layer to act as an amplifier for the incoming Tollmien–Schlichting waves, the intensity and spectral characteristic of the radiated tonal noise can be affected depending upon the serration geometry. Particle Imaging Velocimetry (PIV) measurements of the airfoil wakes for a straight and serrated trailing edge are also reported in this paper. These measurements show that localized normal-component velocity fluctuations that are present in a small region of the wake from the laminar airfoil become weakened once serrations are introduced. Owing to the above unique characteristics of the serrated trailing edges, we are able to further investigate the mechanisms of airfoil instability tonal noise with special emphasis on the assessment of the wake and non-wake based aeroacoustic feedback models. It has been shown that the instability tonal noise generated at an angle of attack below approximately one degree could involve several complex mechanisms. On the other hand, the non-wake based aeroacoustic feedback mechanism alone is sufficient to predict all discrete tone frequencies accurately when the airfoil is at a moderate angle of attack.     

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...     

On the generation of side-edge flap noise

A theory is proposed for estimating the noise generated at the side edges of part span trailing edge flaps in terms of pressure fluctuations measured just in-board of the side edge on the upper surface of the flap. Asymptotic formulae are developed in the opposite extremes of Lorentz contracted acoustic wavelength large/small compared with the chord of the flap. Interpolation between these limiting results enables the field shape and its dependence on subsonic forward flight speed to be predicted over the whole frequency range. It is shown that the mean width of the side edge gap between the flap and the undeflected portion of the airfoil has a significant influence on the intensity of the radiated sound. The results indicate that the noise generated at a single side edge of a full scale part span flap can exceed that produced along the whole of the trailing edge of the flap by 3 dB or more.     

Noise due to turbulent flow past a trailing edge

A theoretical method [I] for calculating far field noise from an airfoil in an incident turbulent flow is extended to apply to the case of noise produced by turbulent flow past a trailing edge, and some minor points of the theory in reference [1] are clarified. For the trailing edge noise, the convecting surface pressure spectrum upstream of the trailing edge is taken to be the appropriate input. The noise is regarded as generated almost totally by the induced surface dipoles near the trailing edge and thus equal, but anticorrelated, noise is radiated into the regions above and below the airfoil wake, respectively. The basic assumption of the analysis, from which these concepts of appropriate input and dominance of dipole sources follow, is that the turbulence remains stationary in the statistical sense as it moves past the trailing edge. The results show that such trailing edge noise often is quite small, compared say to that produced by typical oncoming turbulence levels of one percent, but that it might be appreciable for an airfoil with a flow separation, or for a blown flap.     

Broadband trailing edge noise from a sharp-edged strut

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 10(5) to 5.0 × 10(5)). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.     

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Far field noise data indicated that for practical upper surface blown flap configurations, the noise radiated below the flap is dominated by the noise generated in the vicinity of the trailing edge. The sound field caused by turbulent mixing in the trailing edge wake is investigated experimentally and theoretically. Hot wire measurements were made downstream of the trailing edge to determine the gross turbulent mixing characteristics of the flow. This information is used as input to a theoretical analysis of the sound field. Favorable agreement is found between predicted and measured far field noise directivity at various frequencies and noise power spectra in various directions.     

Experimental and numerical study on aeroacoustic sound of axial flow fan in room air conditioner

Fan is one of the main noise sources of the room air-conditioners. Axial flow fans are widely used in the outdoor unit of split type air-conditioners. The interaction between the fan and the heat exchanger should be taken into consideration. However, only a few researches have been carried out on predicting the aeroacoustic noise because of the difficulty in obtaining detailed information of the flow field. This paper is to understand the generation mechanism of sound and to develop a prediction method for the flow field and the acoustic pressure field of the outdoor unit. Acoustic measurement is performed in a semi-anechoic chamber. Effects of each components is analyzed. Based on commercial computational fluid dynamics (CFD) code, Fluent, Fukano’s model is used to predict the overall sound pressure level of broadband noise. The predicted sound pressure levels based on original Fukano’s model are 7.66 dB and 7.42 dB lower than measurement results at 780 rpm and 684 rpm, respectively. And the errors are about 13%. However, when wake width and relative velocity are both calculated by numerical simulations and the distance to blade trailing edge is taken into consideration, the difference of sound pressure level between measurement and prediction is less than 3.4 dB and errors less than 5.5% while the distance is less than 10 mm. Thus, the distance to blade trailing edge should also be an important parameter for Fukano’s model. In comparison with experimental results, it is clearly shown that the Fukano method based on numerical simulation can provide more accuracy than the original Fukano model and numerical results are in a reliable level.     

Experiments on the unsteady flow field and noise generation in a centrifugal pump impeller

This paper reports on an experimental investigation of large-scale flowfield instabilities in a pump rotor and the process of noise generation by these instabilities. Measurements of the fluctuating components of velocity and surface pressure were made with hot-wire probes and surface mounted pressure transducers on a seven bladed back swept centrifugal water pump impeller operating with air as the working fluid. The impeller was operated without a volute or scroll diffuser, thereby eliminating any sound generation from pressure fluctuations on the volute cutoff. Thus the study focused on flow field and noise components other than the blade passage frequency (and its harmonics). The primary goal of the study was to provide fundamental information on the unsteady flow processes, particularly those associated with the noise generation in the device. It was further anticipated that detailed flow measurements would be useful for the validation of future computational simulations.The measured data at the discharge show a jet-wake type of flow pattern which results in a strong vorticity field. The flow with high velocity found on the pressure side of the impeller tends to move to the low-pressure region present at the suction side of the passage as a form of roll-up around the blade trailing edge. This motion causes an unsteady flow separation at the suction side of the blade and consequently disturbs the flow in the adjacent passage. By interacting with the impeller blades near the trailing edges, this instability flow causes a periodic pressure fluctuation on the blade surface and generates noise by a trailing edge generation mechanism. The spectrum of surface pressure measured at the trailing edge of each blade reveals a cluster of peaks which were identified with azimuthal mode numbers. The correlation between the acoustic farfield pressure and the surface pressure on the impeller blade has proven that the azimuthal modes synchronized with the number of impeller blades generate noise much more efficiently than the other modes. The paper also clarifies the correlation between unsteady flowfield measurements, in both impeller and laboratory co-ordinates, with the radiated noise properties. Thus some light is shed on the noise generation mechanisms of this particular device.