Abstract The tonal noise generation of a circular cylinder in a uniform flow is an important source of aerodynamic noise. It can be found at parts of the landing gear of airplanes, at pantographs of trains, at antennas and basically all other protruding parts of vehicles. This noise is due to the periodic shedding of vortices along the cylinder span. One method to reduce this noise is the use of flow permeable covers around the cylinders. In the present study, measurements were performed in an aeroacoustic wind tunnel on a large set of porous covered cylinders. In addition to varying the porous material, which is characterized by its air flow resistivity and its porosity, the thickness of the porous layer was varied as well. The measurements were performed at Reynolds numbers between 14,000 and 103,000 using microphones located in the acoustic far field. It was found that the porous covers lead to a notable narrowing of the vortex shedding tonal peak in the sound pressure level spectra, an effect that increases with increasing porosity and thickness and decreasing air flow resistivity of the porous layer. Based on the large set of experimental data, basic trends were derived for the estimation of the vortex shedding Strouhal number and the reduction in the energy in the vortex shedding peak using the method of linear regression. Constant temperature anemometry measurements in the wake of selected cylinders basically showed a similar narrowing of the vortex shedding peak in the spectra of the turbulent velocity fluctuations. In addition, the measurement of wake profiles showed a reduction in the mean velocity and the turbulence in the wake as well as a widening of the wake region, while an analysis of the spanwise coherence revealed that the cause of the overall noise reduction is not a breakup of spanwise turbulent structures. Rather, the results imply that viscous damping of turbulent flow pressure amplitudes by the porous material strongly contributes to the noise reduction. Graphic abstract

中文翻译：

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多孔材料圆柱涡脱落降噪实验评价

摘要 均匀流动中圆柱体产生的音调噪声是气动噪声的重要来源。它可以在飞机起落架的部件、火车的受电弓、天线以及基本上所有其他车辆突出部件中找到。这种噪音是由于沿汽缸跨度的周期性旋涡脱落造成的。减少这种噪音的一种方法是在气缸周围使用可渗透的盖子。在本研究中，测量是在气动声学风洞中对大量多孔覆盖圆柱体进行的。除了改变以空气流动阻力和孔隙率为特征的多孔材料之外，多孔层的厚度也改变。测量是在雷诺数介于 14,000 和 103 之间进行的，000 使用位于声学远场中的麦克风。发现多孔覆盖物导致声压级谱中的涡旋脱落音调峰显着变窄，这种影响随着多孔层的孔隙率和厚度的增加以及空气流动电阻率的降低而增加。基于大量的实验数据，利用线性回归的方法推导出涡流脱落斯特劳哈尔数和涡流脱落峰能量减少的基本趋势。在选定圆柱体的尾流中进行的恒温风速测量基本上显示了湍流速度波动频谱中涡旋脱落峰的类似变窄。此外，尾流剖面的测量表明尾流中的平均速度和湍流降低，尾流区域扩大，而展向相干性分析表明，整体降噪的原因不是展向的分裂湍流结构。相反，结果表明多孔材料对湍流压力振幅的粘性阻尼对降噪有很大贡献。图形摘要