气流管道内多模态远场辐射特性研究

Investigation of Multimode Far-field Radiation Characteristics in Airflow Ducts

引入了无量纲多模态指向性函数,针对半无限长、存在均匀亚声速流动的硬壁圆形管道内的多模态噪声辐射特性进行数值和实验研究。将模态的指向性函数表示为远场声压平方与声功率的比值,其值依赖于各个导通模态的幅值和方向函数。数值研究中包含3种声源分布模型:均匀分布的不相干单极子、均匀分布的不相干偶极子以及模态等能量模型。数值研究表明每个模态的幅值可以表示为模态导通比的函数。对均匀流下的多模态指向性函数的精确数值解和解析解进行对比,并在高频处进行验证研究,结果表明在高频处数值解和解析解相当符合。模态指向性函数随着气流马赫数增大而增大,轴流风扇噪声辐射结果在高频处与根据模态等能量模型计算出的远噪声指向性最相符,精度最高。数值和实验结果表明模态等能量声源模型可以用于预测风扇在高频处的远场辐射噪声。

The multimode noise radiation characteristics of a semi-infinite,rigid-walled circular duct with internal uniform subsonic airflow are analyzed numerically and experimentally.The dimensionless multimode directivity function is introduced for analysis.This directivity function is expressed as the ratio of the square of the radiation sound pressure in the far-field to the sound power,which value depends on the amplitude and directivity function of each individual cut-on mode.Three source distribution models are used in the numerical study,such as a uniform distribution of incoherent monopoles,a uniform distribution of incoherent dipoles,and equal energy per mode.The numerical study shows that the amplitude of each mode can be expressed as a function of mode cut-on ratio.This exact numerical result of the multimode directivity function in uniform flow is compared with an analytic solution,and validated in the high-frequency range.And a good agreement between the numerical and analytic results is shown.The results show that the mode directivity function increases with the increasing of the airflow Mach number.The axial fan radiation noise results are excellently consistent with the far-field radiation directivity of equal energy per mode source model with highest accuracy.Equal energy per mode source model is verified numerically and experimentally,which can be used to predict fan radiation noise at high frequencies.