Low-frequency noise from large wind turbines

As wind turbines get larger, worries have emerged that the turbine noise would move down in frequency and that the low-frequency noise would cause annoyance for the neighbors. The noise emission from 48 wind turbines with nominal electric power up to 3.6 MW is analyzed and discussed. The relative amount of low-frequency noise is higher for large turbines (2.3-3.6 MW) than for small turbines (≤ 2 MW), and the difference is statistically significant. The difference can also be expressed as a downward shift of the spectrum of approximately one-third of an octave. A further shift of similar size is suggested for future turbines in the 10-MW range. Due to the air absorption, the higher low-frequency content becomes even more pronounced, when sound pressure levels in relevant neighbor distances are considered. Even when A-weighted levels are considered, a substantial part of the noise is at low frequencies, and for several of the investigated large turbines, the one-third-octave band with the highest level is at or below 250 Hz. It is thus beyond any doubt that the low-frequency part of the spectrum plays an important role in the noise at the neighbors.     

Long time measurements of noise from wind turbines

Noise is an important environmental factor at windmill sites and hence there is a need for reliable methods to control the exposure situation. It is difficult and time consuming to make field measurements that cover a large variety of metrological situations. A measurement station for long time noise measurements have been developed which makes it possible to make noise registrations and recordings at different weather situations.     

CAA broadband noise prediction for aeroacoustic design

The current status of a computational aeroacoustics (CAA) approach to simulate broadband noise is reviewed. The method rests on the use of steady Reynolds averaged Navier-Stokes (RANS) simulation to describe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulence kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations having statistics that very accurately reproduce the initial RANS target-setting. The synthetic fluctuations are used to prescribe sound sources which drive linear perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art CAA tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the random particle-mesh (RPM) and fast random particle-mesh (FRPM) method is given. Results are presented for trailing-edge noise, slat noise, and jet noise. Some problems related to the formulation of vortex sound sources are discussed.     

基于数据融合的风电机组噪声预测

根据风电机组噪声信号检测复杂的情况,研究风电机组非声学参数的信息熵特征,对机组噪声进行多源数据融合预测。分析基于信息熵的非声学参数的特征提取方法,并对传统的基于遗传算法的支持向量机回归(GA-SVR)的缺陷提出改进,结合实际应用的非声学参数的信息熵特点平衡遗传算法(GA)的终止条件。通过统计分析完成了输入变量的筛选,去除了对预测影响较大的共线性因素,并实现了输入降维提高预测精度和速率。最后应用数据的信息熵特征,训练改进的GA-SVR建立最终的多源数据特征级融合预测模型。通过对比表明基于多源数据融合的预测方法精度最高,预测结果的相对误差平均值为0.7757%,具有实际可行性。     

Improved prediction of the turbulence-shear contribution to wind noise pressure spectra

In previous research [Raspet et al., J. Acoust. Soc. Am. 123(3), 1260-1269 (2008)], predictions of the low frequency turbulence-turbulence and turbulence-mean shear interaction pressure spectra measured by a large wind screen were developed and compared to the spectra measured using large spherical wind screens in the flow. The predictions and measurements agreed well except at very low frequencies where the turbulence-mean shear contribution dominated the turbulence-turbulence interaction pressure. In this region the predicted turbulence-mean shear interaction pressure did not show consistent agreement with microphone measurements. The predicted levels were often much larger than the measured results. This paper applies methods developed to predict the turbulence-shear interaction pressure measured at the ground [Yu et al., J. Acoust. Soc. Am. 129(2), 622-632 (2011)] to improve the prediction of the turbulence-shear interaction pressure above the ground surface by incorporating a realistic wind velocity profile and realistic turbulence anisotropy. The revised prediction of the turbulence-shear interaction pressure spectra compares favorably with wind-screen microphone measurements in large wind screens at low frequency.     

自适应宽带有源消声

实际中存在的噪声,一般都是窄带或有色宽带噪声(简称宽带噪声),而宽带噪声更为普遍.为使宽带自适应有源消声(AANC)得到实际应用,必须保证宽带AANC系统具有良好的稳定性和较高的降噪量.为此,本文对AANC系统稳态性能作了理论分析和数值计算,得到了系统降噪量对噪声带宽、空间声传播通道、自适应滤波器参数等的依赖关系;以自由声场远场AANC为例,从声学角度对AANC系统作了物理解释,从而为改进宽带AANC系统性能提供理论依据.     

On the prediction of impact noise,V: The noise from drop hammers

In earlier papers in this series, the concepts of “acceleration” and “ringing” noise have been studied in relation to impact machines, and values of radiation efficiency have been obtained for the various types of structural components. In the work reported in this paper the predicted and measured noise radiation from a drop hammer, both in full-scale and in $\text{1}\text{3}\text{-}\text{scale}$ model form, were examined. It is found that overall noise levels (L_eq per event) can be predicted from vibration measurements to within ± 1·5 dB, and to within ±2·5 dB in one-third octave bands. In turn this has permitted noise reduction techniques to be examined by studies of local component vibration levels rather than overall noise, a method which provides considerable enlightenment at the design stage. It is shown that on one particular drop hammer, the noise energy is shared surprisingly uniformly over four or five sources, and that when these have been reduced, the overall noise reduction is severely limited by the “acceleration” noise from the “tup” or “hammer” itself. As this is difficult to eliminate without a basic change in forging technology, it follows that “tup” enclosure or modification of the sharpness of the final “hard” impact are the only means available for any serious noise reduction. Also indicated is the reliability of using model techniques, suitably scaled in frequency and impulse magnitude, in developing machinery with impact characteristics.     

Cyclostationary spectral analysis for the measurement and prediction of wind turbine swishing noise

This paper introduces cyclostationary spectral analysis as a new approach to analyzing and predicting the aerodynamic noise generated by wind turbines. This method is able to reveal new insights into the periodic character of the noise signal and is therefore ideally suited to the study of wind turbine noise. A new formulation is presented for the time variation of the noise spectrum due to wind turbines thereby providing insight into the character of the periodic variation in noise referred to as ‘swishing’. The character and mechanism of swishing noise is analyzed in detail.     

On the prediction of impact noise,Part IX: The noise from punch presses

The punch press is a further classical example of impact noise: the force is built up in the punch relatively slowly, and the whole machine is strained until the load on the material being punched reaches a yield and shortly afterwards a fracture level. At this stage the strain energy in the machine and its workpiece must be redistributed and this leads to vibration of the whole machine and to noise radiation. In this paper work which has been carried out at ISVR over a number of years is presented. It describes the way the Energy Accountancy Equation [1] can be modified to relate the noise radiated directly to the sum of the squares of the large rates of change of force against time, and illustrates clearly the way that noise control, with use of passive or active methods in designing the punch tooling, can be related directly to the one parameter 10 log Σ[f(t)]_max². It is shown that noise levels can be reduced by up to 30 dB under ideal conditions by fracture pulse tailoring while still cutting metal, but that more realistically, 10 dB reductions may be obtained below those of current practice by practical tool design. An explanation is given of why large open presses are never likely to achieve the proposed factory noise levels and that the noise from double-sided presses can be controlled effectively by tooling modifications and by the addition of damping. The paper describes work carried out on passive and active cancellation systems used to arrest and springback of the press structure following workpiece material fracture and explains the practical limitations of such systems. Active techniques are limited by the difficulties obtained, with brittle hard materials, of finding a trigger and load freezing system which can operate fast enough to reduce adequately the force parameter 10 log Σ (f_max(t))². Where punching finish allows, it has been found that well-designed shear and/or cutting with low percentage clearance is superior to active cancellation, since this provides a surer method of reducing the above force parameter, especially for harder materials. The Energy Accountancy Equation suggests that the use of increased structural damping can be an effective method of noise reduction, particularly if the punch press structure is initially only lightly damped. Experiments on a third-scale model of a 200 tonne double-sided press suggest that this damping needs to be added selectively in areas where the vibrational energy density is high and coincides with areas of high noise radiation. Thus, in the experiments described, 12 dB reduction has been obtained under such conditions, only 1 or 2 dB reduction being obtained when damping of the base plate has been unaltered.     

The prediction of noise from urban traffic under interrupted flow conditions

A computer model is described for predicting noise levels generated by urban road traffic under interrupted flow conditions. The model is composed of two subsections. The first predicts the propagation characteristics of sound in typical street configurations and the second simulates the flow of road traffic in urban areas. The two subsections are combined to yield a model capable of predicting traffic noise levels in urban conditions.Predictions obtained from application of this model are compared with those given from application of predictive models based upon field measurements. The agreement between the predictions is good. It is shown that the model described in this paper can predict noise levels for situations which existing field-based models cannot handle.     

Sharpness and amplitude envelopes of broadband noise

This paper discusses how amplitude envelope shapes, sound-pressure level, and duration of broadband noise affect sharpness and brightness. In the first experiment, sharpness, brightness, and similarity were judged for paired stimuli by 13 subjects. The stimuli consisted of broadband noise with different types of amplitude envelopes, sound-pressure levels, and duration. Experimental results were analyzed with a multidimensional scaling technique. In addition, the second experiment measuring the point of subjective equality (PSE) of sharpness was carried out for four subjects. The results of these two experiments showed large individual differences in sharpness judgment, difficulty in brightness judgment for the stimuli used in the experiments, and influence of the amplitude envelope shapes on sharpness. The individual differences observed in sharpness judgment were explained by the differences between weights given to two psychological dimensions: loudness and subjective duration. This study forms a basis for understanding sharpness of nonsteady sounds.     

A novel feedback active noise control for broadband chaotic noise and random noise

The feedback active noise control (ANC) can be seen as a predictor, the conventional method based on filtered-x least mean square (FXLMS) algorithm can only be useful for linear and tonal noise, but for nonlinear and broadband noise, it is useless. The feedback ANC using functional link artificial neural networks (FLANN) based on filtered-s least mean square (FSLMS) algorithm can reduce some nonlinear noise such as chaotic noise, but the noise cancellation performance is not very well, at the same time, it is not useful to random noise. To solve the problem above, a new feedback ANC using wavelet packet FXLMS (WPFXLMS) algorithm is proposed in this paper. By decomposing the broadband noise into several band-limited parts which are predictable and each part is controlled independently, the proposed algorithm can not only suppress the chaotic noise, but also mitigate the random noise. Compared with FXLMS and FSLMS algorithms, proposed WPFXLMS algorithm also holds the best performance on noise cancellation. Numerous simulations are conducted to demonstrate the effectiveness of the proposed WPFXLMS algorithm.     

煤矿大型风机设备磨损监控系统设计

针对煤矿大型风机设备主轴磨损而影响风机工作效率的问题,在对大型风机设备磨损故障的生成原因分析后,根据系统需求设计实现了系统的硬件模块和软件模块,通过传感器电路对大型风机设备的输入输出功率、出风量、主轴转速等参数进行采集,系统软件在对数据进行综合分析后将检测结果显示在液晶屏上,实时输出大型风机设备的运行状态以及主轴磨损状况,从而保证了风机的运行效率,避免设备故障的发生。通过系统测试表明本系统可以实时输出风机设备的主轴磨损程度监测结果,且风机设备的风量、风压、功率和效率等主要参数的测量精度高。     

Prospects for development of wind turbines with orthogonal rotor

The experimental data obtained previously on the investigation of power characteristics and the possibility of the self-start of the Darrieus rotor are anlysed. These results are used at the design of new two-tier wind turbines with straight blades. The full-scale tests of two design variants showed the prospects for the development of wind turbines with the Darrieus rotor. At a reasonable design, they do not need any devices for the rotor orientation and start-up, are little sensitive to wind gusts and can have a high level of power characteristics, which is not inferior to the best samples of the units of propeller type.     

Fan broadband noise shielding for over-wing engines

Increasingly demanding community noise targets are promoting noise performance ever higher on the list of airliner design drivers. In response, significant noise reductions are being made, though at a declining rate—it appears that a whole airframe approach is now needed to achieve significant further gains. As a possible step in this direction, over-wing engine installations are considered here, which use the airframe itself as a noise shield. The paper is the account of an experimental investigation of the comparative shielding performances of a range of relative engine positions on such a layout. Using the statistical modelling technique Kriging, we build an approximation of the noise shielding metric as a function of the position of the engines above the wing—this can serve as the input to multi-disciplinary design trade-off studies. We then compare the results found with the results of applying simple half-barrier diffraction theory to the same problem. We conclude that the latter could be considered as a first order, conceptual design tool, though it misses certain features of the design merit landscape identified by the experiment presented here.