An Investigation of the Genaration Mechanism of Discrete Frequency Noise Relating to the Large Scaled Turbulent Organized Structures and Its Noise Control

确定了二维平板翼型在均匀流动中发出离散频率噪声的流场，并对噪声的产生机理进行了研究。采用频率分析和锁相平均技术测定了流场。采用i型和x型热丝探头获得尾迹速度场，采用安装在翼型上的小型半导体压力传感器测量翼型表面的脉动压力，并在近尾迹区插入小直径圆柱体对发射噪声声级进行人为调整。通过对尾流中有组织结构和翼型表面脉动压力等流动细节的定量分析，研究了尾流中大型有组织结构离散频率噪声的产生机理。湍流尾流中清晰连贯的组织结构在每种情况下都得到证实。离散频率噪声的声源分布在翼型训练边缘附近的狭窄区域内。定量地证实了声源是由尾迹湍流组织结构引起的翼型上的速度波动所引起的脉动压力所产生的。湍流尾迹中有组织结构的跨向相关性对离散频率噪声的声压级有主要影响。对声强测量系统进行了标定，结果表明，声源在三维空间内的定位精度在1mm以内。

Flow fields around a two-dimensional flat plate aerofoil emitting discrete frequency noise in a uniform flow have been determined, and the generation mechanism of the noise has been studied.Frequency analysis as well as phase-locked averaging technique were used for determination of flow fields. The wake velocity fields were obtained with I-type and X-type hotwire probes and the fluctuating pressure on the aerofoil surface was measured with small-sized semiconductor pressure transducers mounted in the aerofoil, whereas the sound level of emitted noise were artificially modified by inserting a circular cylinder with a small diameter in the near wake region. The flow details, such as organized structures in a turbulent wake flow as well as fluctuating pressures on the aerofoil surface, have been analyzed quantitatively, from which the generating mechanism of discrete frequency noise has been studied relating to the larged scaled organized structures in a turbulent wake.Clear coherent organized structures in a turbulent wake are confirmed in every case. The sound source of discrete frequency noise are distributed in a narrow region restricted near the traihing edge of the aerofoil. It has been quantitatively confirmed that the sound sources are generated from the fluctuating pressures due to the fluctuating velocities on the aerofoil induced from the turbulent organized structures in the wake. The span-wise correlations of the organized structures in the turbulent wake have a dominant effect on the sound pressure level of the discrete frequency noise.The calibratrion of the sound intensity mesuring system has been made, which shows that the location of the sound source can be determined within the accuracy of 1mm in the three-dimensional space.