Excitation and global calibration of duct modes with high orders and complex mixtures inside a cylindrical duct with flow
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摘要: 声模态发生器是通过调控扬声器阵列，进而在管道内激发特定阶次声模态的一种装置，可用于流管内气动噪声传播和环形声衬的降噪研究。使用多圈布置的扬声器阵列模态发生器，通过调节各个声源的幅值和相位，可实现高阶周向、径向和混合模态的激发。然而，扬声器阵列系统的通道输出偏差会在目标模态激发时产生显著的干扰模态。为消除扬声器系统通道输出差异对模态激发的影响，提出了基于最小二乘全局标定的流管内高阶混合模态激发方法。通过对扬声器声源的通道输出偏差进行全局建模，引入激励补偿因子和流场校正因子，将有流情况下各扬声器的系统输出偏差求解问题转换为无流情况下单个扬声器工作时的模态识别问题，通过矩阵变换和最小二乘法求解各扬声器的激励补偿因子，最终实现扬声器的幅值和相位激励修正。将所发展的模态激发与全局标定方法应用于所研制的圆形流管模态激发装置，结果表明：干扰模态的强度被显著抑制，且在工作频率范围内，目标激发模态的相对目标模态系数不低于10 dB。Abstract: The spinning mode synthesizer is of great significance to the study of the propagation and radiation of aerodynamic noise in the cylindrical duct with flow as well as the evaluation of noise reduction with sound liners, working by controlling the array of loudspeakers and generating the specific distribution of the acoustical mode inside the duct. High-order circumferential modes, radial modes and their mixtures should be excited accurately with the manipulation of the multiple-rings array of loudspeakers. However, due to the systematic error of the loudspeaker, significant interference modes are generated simultaneously with the target mode, remarkably affecting the accuracy of mode excitation. A mode excitation method based on the least square and global calibration is proposed to motivate the target duct mode inside a flow duct and eliminate the influence of loudspeakers' system error on mode synthesizing. Through the global modeling of the system error of the loudspeaker sound source, the global calibration factor and flow field correction factor are introduced to convert the problem of solving the system error of each loudspeaker in the case of the flow field into the mode identification problem of a single loudspeaker in the case of no flow. The complex calibration factor of each loudspeaker is solved by matrix transformation and the least square method, after which the amplitude and phase excitation correction of the loudspeaker is realized. The presented method is applied to the spinning mode synthesizer in SJTU, whose experimental results indicate that the intensity of the interference modes is significantly suppressed, and the modal coefficient signal-to-noise ratio (SNRA) of the target excitation modes is greater than 15 dB within the operating frequency range.
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Key words:
- cylindrical flow duct /
- spinning mode synthesizer /
- duct mode /
- system deviations /
- global calibration
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图 1 轴向多圈布置的管壁声源阵列
Figure 1. An axial multi-rings source array arranged on the duct wall
图 2 激励补偿因子的测量与模态激发过程
Figure 2. The process of calibration and duct mode excitation in the flow environment
图 8 流速19.85 m/s下 3300 Hz激发的(7, 0)阶模态声源复强度
Figure 8. The complex source strength for mode (7, 0) in 3300 Hz at a flow velocity of 19.85 m/s
图 9 流速19.85 m/s下 3700 Hz 激发的(0, 2)和(5, 0)阶模态的声源复强度
Figure 9. The complex source strength for modes (0, 2) and (5, 0) in 3700 Hz at a flow velocity of 19.85 m/s
图 10 流速19.85 m/s下 3300 Hz 激发的（7, 0）阶模态的识别结果
Figure 10. Mode calculation results for mode (7, 0) in 3300 Hz at a flow velocity of 19.85 m/s
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