In this study, the active control of aerodynamic noise generated by flow around a circular cylinder was investigated numerically at a diameter-based Reynolds number of Re = 4.7 x 10(4) using a forced rotation method. The effects of rotation rates (i.e., the ratio of tangential velocity to free-stream velocity) ranging from alpha = 0 to 5 on aeroacoustic and aerodynamic characteristics, as well as flow patterns, were comprehensively analysed to uncover the mechanisms of noise reduction. Three-dimensional large eddy simulation, coupled with the Ffowcs Williams-Hawkings equation, was employed to accurately capture the evolution of flow characteristics and acoustic fields. The potential influence of this technique on noise reduction was demonstrated across the entire range of alpha, with variation observed within four distinct regimes: I (alpha = 0-0.6), II (alpha = 0.6-1), III (alpha = 1-2.5), and IV (alpha = 2.5-5). It was shown that the best performance in tonal noise reduction, approximately 26 dB, was achieved in Regime III, particularly at alpha = 2.5. The primary mechanism for noise reduction was identified as the suppression of Kelvin-Helmholtz instability within the shear layers. This suppression reorganised the shear layers, breaking down large-scale turbulent structures into small-scale fluctuations that dissipate rapidly. Moreover, the coherence between dominant modes was suppressed, further weakening organised vortex interactions. Consequently, vortex-induced noise from the cylinder was effectively mitigated.

To satisfy the vibration and noise control requirements for cabins, sail, and other structures in ship equipment, this paper focuses on the design and fabrication of the high specific stiffness of grille composite structures and the significant damping characteristics of the core layer. Modal and vibro-acoustic tests verify that the foam-filled grille composite exhibits vibration and noise reduction effects, while validating the accuracy of the Finite Element Method (FEM) combined with Boundary Element Method (BEM). It shows that the overall acceleration levels of monitoring points decreased by 3.61 dB and 0.76 dB, and the overall sound pressure levels decreased by about 4.69 dB and 4.76 dB, respectively. Then the grille composite sandwich panels and an equal-density steel panels are compared for force-induced vibration sound radiation with different layup sequences, core filling materials, excitation positions, and damping ratios in both air and water environments. Research demonstrates that grille composite sandwich structures exhibit superior specific stiffness and damping characteristics compared to traditional metallic materials, resulting in enhanced vibration and noise reduction performance.

This study proposes a low-frequency broadband meta-liner (extended-slit resonator meta-liner, ERML) based on an extended slit-neck resonator and a perforated panel. A theoretical impedance model under grazing flow and a semi-analytical numerical model with equivalent impedance boundary conditions were established. The noise reduction performance and sound absorption characteristics under different typical low Mach number grazing flow conditions were comprehensively investigated. The ERML achieves broadband noise reduction below 500 Hz at grazing flow velocities ranging from 0 to 0.15 Ma, achieving the thickness-to-wavelength ratio of 1/24. Under static conditions, the measured average transmission loss within 150-500 Hz reaches 7 dB, while at Ma = 0.15, the corresponding value is 6.2 dB, suggesting that the presence of grazing flow reduces the sound attenuation performance of ERML. This reduction in sound attenuation can be attributed to the interaction between the grazing flow and the meta-surface, which affects the acoustic energy dissipation capability of ERML, leading to a slight decline in overall efficiency. Comparative analysis confirms that the integration of a perforated panel stabilizes the acoustic performance of ERML under grazing flow, enhancing its adaptability to complex flow fields. Finally, experimental samples were fabricated and tested in a flow duct to evaluate acoustic performance under grazing flow, validating the effectiveness of the theoretical model and numerical method. The ERML demonstrates outstanding low-frequency broadband noise suppression, strong adaptability to actual flow environments, along with advantages in structural simplicity and manufacturability, exhibiting promising prospects for noise control applications.

This study proposes a passive noise reduction method using cylindrical leading-edge perturbation structures in the bogie region of high-speed trains. Large Eddy Simulations combined with the Ffowcs Williams–Hawkings acoustic analogy were performed on a 1:8 scale simplified bogie model at 400 km/h to assess the effectiveness of this approach. Cylindrical leading-edge structures with diameters of 0.5δ, 0.625δ, 0.75δ, and 0.875δ (δ: local boundary layer thickness) were assessed for their impact on flow disturbances, Spectral Proper Orthogonal Decomposition modes, dipole source power, and far-field noise. The 0.875δ cylindrical leading-edge structure achieved the greatest noise reduction, lowering far-field Overall Sound Pressure Level by up to 3.7 dB. This reduction is primarily due to the suppression of tonal peaks near 250 and 500 Hz. The 250 Hz tonal peak primarily originates from a large-scale recirculating feedback flow (L1) within the bogie cavity. The cylindrical leading-edge disturbance modifies the shear layer separation angle, reduces flow impingement on the rear cavity wall, and displaces the recirculation zone (L2) downward, thereby weakening the tonal feedback loop. The 500 Hz tonal peak arises from strong dipole sources observed near the front axle, bogie frame, and the lower surfaces of both front and rear wheels. The cylindrical leading-edge disturbance modifies the dominant flow modes, reducing their interaction with the axle and bogie frame and thereby disrupting flow-structure coupling. This results in a substantial reduction in noise energy, with small-scale feedback structures (S1 and S2) nearly eliminated and the intensity of S3 significantly reduced.

This study proposes a type of sound-absorbing metastructure consisting of micro-slits panel (MSP) and extended slit-neck Helmholtz resonator (HR) to achieve efficient sound absorption in the low-frequency range. Based on the theory of thermal viscosity and double-slit parallel coupling, an analytical method for predicting the sound absorption performance of the proposed metastructure is established and validated by comparison with finite element simulations and experiments. The absorption mechanism and influences of parameters are analyzed. It is revealed that the metastructure can meet the requirements of efficient impedance modulation at low frequencies. The velocity gradient of particle vibration between slits and the wall leads to the viscous dissipation of sound energy. Furthermore, optimization design of the metastructure is conducted, it is demonstrated that an optimized design (60 mm thickness) with only four unit cells can achieve efficient absorption (alpha > 0.9) in the low-frequency broadband range of 372 Hz-814 Hz. This work provides a new pathway for low-frequency and broadband sound absorption design of metastructures with simple construction and low manufacturing cost.

Experimental investigations are performed to study the trailing edge noise reduction over NACA (National Advisory Committee for Aeronautics) 0018 airfoil using 3D-printed porous trailing-edge inserts with heterogeneous distribution of pores. Cylindrical channels are distributed over 19 inserts of 20% chord length. Twelve inserts with homogeneous and the other seven with heterogeneous porosity distributions are tested at seven chord-based Reynolds numbers ranging from 2.09 × 105 to 6.27 × 105 and three effective angles of attack of 0.2°, 2.0°, and 4.3°. For the homogeneous models, the porosity, pore diameter, and patterns of distribution are investigated parametrically to study their effects on noise reduction. For the heterogeneous models, various streamwise porosity gradients are introduced, which effectively delay the onset velocities of tonal noise and reduce high-frequency excess noise due to surface roughness. An optimized model is subsequently proposed to provide a smooth transition of porosity at the solid-porous juncture and the trailing edge, which results in broadband noise mitigation within the entire tested frequency range. Smaller variations in noise reduction capability are obtained at different flow conditions. Using the dataset of homogeneous and heterogeneous porous models, we propose an empirical scaling function for the sound pressure level difference, which shows good agreement with the tested results and can therefore be used for future optimal noise reduction designs.

The exterior noise of large aircrafts is an important aspect for the airworthiness certification. The high-lift device is one of the key noise sources for large aircrafts, thus the analysis and reduction of high-lift device noise is important and meaningful. Based on the 5.5m×4.0m Aeroacoustic Wind Tunnel and a half model for the aeroacoustic study of large scale aircrafts, the high-lift device noise properties were studied experimentally. First, the experimental platform, the test model, the test system and noise reduction design were introduced. Then, based on the experimental results, the noise characteristics of the high-lift devices were analyzed. The effects on the noise from the angle of attack and the wind speed were compared. The effects on the high-lift device noise from the noise reduction design on slat and flap were investigated. The results show that, the high-lift device noise has a broad frequency spectrum, with some peak frequency components. As the angle of attack is increased, the noise of the model is generally increased in the medium to high frequency range. With the wind speed increased, the noise is increased dramatically, and the frequency spectra keep a good similarity. The peak noise is suppressed by the noise reduction design.

Regarding applications of the large-scale aeroacoustic wind tunnel in scientific research, this paper introduces the usage of FL-17, a 5.5 m × 4 m aeroacoustic wind tunnel, in a Sino-Russia joint collaboration research project on civil aircraft landing gear noise and its control. First of all, the construction history and specifications of FL-17 wind tunnel are introduced. Then, based on the large-scale wind tunnel test of landing gear aerodynamic noise in Sino-Russia joint research, the state-of-the-art of the landing gear noise investigation was introduced, as well as the research contents and achievements of the landing gear noise and its control carried out in FL-17 wind tunnel. The discussions involve large-scale experimental studies, database of full-scale landing gear experiment, numerical simulations, prediction models, noise control using truncated cylinders and air curtains, etc. As last, some experiences and suggestions on the usage of large-scale aeroacoustic wind tunnel were provided.

In order to explore more efficient damping structure design,based on the theory of damping vibration and noise reduction technology and artificial cycle structure,the vibration and acoustic radiation characteristics of plate structures with periodic free damp-ing design were studied.Firstly,the finite element simulation model of the acoustic and vibration response of plate structures were es-tablished.Secondly,the vibration and acoustic radiation characteristics of the plates with periodic free damping and traditional free damping were compared,and the influence of different parameters on the effect of periodic free damping were analyzed.Finally,rele-vant experimental research and simulation results verification were carried out.The results show that compared with traditional free damping design,the periodic free damping design can obtain better vibration and noise reduction effect,and help to realize the light-weight design of the damping structure,but the acoustic radiation efficiency will also be changed.The selection of damping materials with high elastic modulus,reasonable design of damping unit shape and appropriate reduction of damping units sectional area can help to improve the performance of periodic free damping design,while the change of the paste angle of the damping unit has little impact on its effect.In addition,the experimental results verify the reliability of the results from simulation study.

In order to solve the problem of flow-induced vibration suppression in the ventilation pipeline, a local resonance periodic pipeline system with low frequency and broadband vibration band gap was constructed. Semi-analytical method and finite element method were used to calculate the energy band structure and vibration transmission loss of the local resonance periodic pipeline, reveal the formation mechanism of the band gap, and analyze the influence of key structural parameters on the band gap characteristics. The vibration reduction and isolation performance test platform of the local resonance periodic pipeline system was built, and the vibration frequency response curve was measured. The research showed that the vibration propagation of the local resonance periodic pipeline could be effectively suppressed in the band gap frequency range. a new idea for the vibration suppression of the ventilation pipeline was provided in this study.

声源区域积分算法是风洞试验中提取飞机组成部件气动噪声源特征的有效数据处理方法.传统声源区域积分算法的积分区域固定,但是飞机机体的气动噪声分布特征会随频率发生明显变化,导致积分结果存在较大误差.为提高频域分布特征变化的声源积分结果准确性,提出了基于CLEAN-SC算法的频域自适应区域积分算法,核心思想是将声源积分区域离散划分,依据CLEAN-SC算法得到的子区域内最强声源位置进行积分区域的自适应优化,从而获取更准确的声源积分结果.通过仿真计算和声学风洞试验数据分析,频域自适应区域积分算法能够得到更为准确的声源积分结果,对于机体气动噪声等动态声源具有更好的适用性.

为解决双轴同步平移机构运动过程中,双轴之间的同步协调控制问题,减小同步误差.本文提出了一种结合滑模PID控制和交叉耦合控制的同步控制方法.首先根据机构结构和运动特点分析了耦合同步误差并建立了系统模型,然后设计了滑模PID控制器来提高双轴运行稳定性,同时采用交叉耦合控制方法将同步误差作为反馈来修正速度给定.最后通过机构实际运动进行了实验对比验证,实验结果表明该控制方法相对普通PID控制方法可有效减小位移同步误差,提高同步性能.该方法也为类似机构的同步控制提供了参考.

在多轴同步系统中,同步控制的目标包括降低同步误差,减小跟踪误差.针对普通偏差耦合控制在实际应用中的问题,对其结构进行改进.该结构通过引入虚拟主轴,虚拟主轴在参考各轴反馈转速的同时,同步完成各轴转速给定补偿,优化了同步控制性能,简化了转速补偿结构.然后,通过结合模糊补偿,可自适应修正控制参数.最后,通过机构运动进行了试验验证,试验结果表明该控制方法相对传统偏差耦合控制结构,在简化系统结构的同时,可进一步提高同步控制性能,在存在负载扰动的运行工况下,可有效降低跟踪误差.该方法也为相关领域的同步控制提供了参考.

The exterior noise of large aircrafts is an important aspect for the airworthiness certification. The high-lift device is one of the key noise sources for large aircrafts, thus the analysis and reduction of high-lift device noise is important and meaningful. Based on the 5.5 m×4.0 m Aeroacoustic Wind Tunnel and a full model for the aeroacoustic study of large scale aircrafts, the high-lift device noise properties were studied experimentally. First, the experimental platform, the test model and the test system were introduced. Then, based on the experimental results, the noise characteristics of the high-lift devices were analyzed. The effects on the noise from the angle of attack and the wind speed were compared, and the directivity properties of the noise propagation were investigated. The results show that, the high-lift device noise has a broad frequency spectra, with some peak frequency components. As the angle of attack is increased, the noise of the model is generally increased in the medium to high frequency range, and the effect of angle of attack varies with the wind speed. With the wind speed increased, the noise is increased dramatically, and the frequency spectra keep a good similarity. In both the lateral and vertical sections, the high-lift device noise propagation shows some directivity characteristics.

声学超材料具有亚波长带隙,可应用于结构振动与噪声控制.引入压电材料和谐振分流电路,可以利用电磁振荡和压电材料机电耦合特性在超材料内部形成可调谐局域共振带隙.传统压电声学超材料受到已有压电材料机电耦合系数的限制,带隙一般较窄,无法满足大柔性结构振动与噪声控制中低频宽带需求.因而,该文提出一种谐振放大压电声学超材料梁结构.将压电片划分为传感极和驱动极,传感极输出电压经过运算放大电路放大,然后与谐振电路相连,实现局域共振效果的增强,从而增大带隙宽度.采用有限元方法建立压电声学超材料梁带隙计算模型,分析了带隙位置和宽度随电路放大倍数的变化,研究了等效弹性模量与带隙的关系;利用商用有限元软件仿真分析了有限周期梁的振动传递特性,验证了带隙计算方法的正确性.研究结果表明,放大电路有效增强了电路的局域共振效果,随着放大倍数的增大,带隙频率降低,带隙宽度增大.

基于相位传声器阵列在声学风洞中测量了低速空腔流动中主要噪声源的分布特性,提出了一种新的评估气动声源强度的计算方法.通过在空腔前后缘加装锯齿板和阻挡板的方式探索了空腔噪声抑制策略,并且比较了不同措施的降噪效果.结果 表明:低速空腔流动噪声主要来自于剪切层与空腔后壁的相互作用;锯齿板以沿来流方向安装在前缘对空腔噪声控制效果最好,且锯齿越密集,噪声抑制效果越明显;阻挡板对空腔噪声控制效果不理想.

Porous material-inspired noise reduction has been proved to be an effective method to reduce airfoil self-noise during the last few decades. However, the used porous materials in previous studies generally have homogenous physical properties. In order to obtain higher airfoil self-noise reduction, in this paper, nonhomogenous porous trailing edges are proposed, which feature a piecewise gradient distributed property in pores per inch (ppi). Detailed acoustic measurements performed in the 0.55 x 0.4-m aeroacoustic wind tunnel showed that the proposed gradient distributed porous trailing edges can achieve a peak noise reduction of up to 43.68 dB and a maximum decrease in overall sound pressure level of up to 19.72 dB, mainly due to the mitigation of vortex-shedding noise of the laminar flow. For turbulent flow, the noise-reduction ability is weakened but can still reduce up to 5.25 dB of turbulent broadband noise. A parameter study of gradient distributed properties on noise reduction revealed several design principles for the proposed modifications, i.e., the porous treatments with larger porous coverage and located at the last 5% chord length of the airfoil gain better noise reduction. Moreover, airfoil self-noise reduction is enhanced when the porous treatments provide a gradual transition from the solid main body to that of the downstream free flow. (C) 2021 American Society of Civil Engineers.

通过对主减速器支撑结构进行周期隔振设计,开展舱内噪声抑制研究.首先,设计了两种周期隔振结构,建立了周期隔振结构的声振分析模型以及作封闭腔体简化的直升机舱室模型;其次,开展了周期隔振结构与均匀连续隔振结构对比研究,分析了周期性设计对隔振结构自身振动传递、单层隔振系统声振响应以及舱内噪声的抑制效果.研究表明:相对于采用非周期隔振设计的情况,周期性设计使结构振动与噪声得到了显著抑制;通过周期隔振结构优化设计,可以在不增加附加质量的情况下,取得更好的减振降噪效果.

Pipe systems are broadly used in modern buildings and large transportation vehicles. The reduction of noise from pipes is one of the main concerns in their engineering applications. In this paper, the reduction of the radiated noise from pipes is studied experimentally using various attached structures. An experimental system is constructed in an anechoic room. Three types of attached structures are considered, including foam coatings, distributed absorbers and periodic absorbers. Via the comparison of basic pipe and pipes with different noise control designs, the reduction effects from the attached structures are compared and analysed. The results show that the radiated noise is effectively reduced by the foam coatings and distributed absorbers, especially, at most of the peaks of the noise. The distributed absorbers reduce the noise more on the whole. The low frequency peaks (below 200Hz) of the noise are reduced most by the periodic absorbers. Via the proper design of the structure types and corresponding parameters, the low frequency property, the broadband property and the reduction amplitude will be obviously enhanced.

The vibration and sound properties of a type of metamaterial sandwich panels are investigated in this paper. The metamaterial sandwich panels consisting of a host sandwich panel and periodically attached resonant units are designed. Both the panels with and without damping are considered. Via the comparison of metamaterial and bare panels, the effects of the periodic design on wave propagation, vibration, sound radiation and sound transmission properties are analysed and compared numerically. The reduction on the vibration and sound is studied. The numerical results indicate that the vibration, sound radiation and sound transmission are significantly reduced over a wide frequency range. The reduction is obviously larger than that obtained only by increasing the mass. In addition, the experiment specimens of bare and metamaterial sandwich panels are designed. The vibration and sound properties of them are tested and compared. According to the experimental results, the reduction is also observed in a wide frequency range. The simulation results and corresponding analysis are verified. Further, the effects of structural parameters of sandwich panels on the reduction of vibration and sound properties from periodic design are investigated. Several typical cases are analysed concretely. The reasons for the reduction and other effects on the vibration and sound properties from periodic design are analysed. For the panels with various parameter settings, a nice reduction of vibration and sound is generated also form periodic design; on the other hand, the reduction characteristics are changed.