Noise Generation and Its Control of Discrete Frequency Noise Emitted from Aerofoils in Consideration of Flow Transition and Large Scaled Coherent Structures

众所周知，在实际应用中，即使是经过精心设计的均匀流型，在某些特定条件下也会发出离散频率噪声，但其产生的机理尚不清楚。本文对传统翼型附近的相干流结构进行了研究，以研究其噪声产生的机理，并为其控制提供有用的信息。本文考虑了以下三种相干流动结构：(a)边界层中由层流过渡到湍流的相干流动；(2)翼型表面局部分离的相干流动；(3)翼型尾迹区形成的大规模湍流相干流动结构。利用FFT对试验翼型前后的速度场进行了分析。利用噪声本身产生的参考信号，用锁相平均技术对其进行了分析。该程序使我们能够识别引起离散频率噪声的相干流。得到的结果可以总结如下：1。对于发出离散频率噪声的翼型，不形成任何流动分离或任何相干尾流。因此，局部分离引起的相干流和相干尾流不是离散频率噪声的产生原因。当发出离散频率噪声时，在靠近翼型后缘的压力面边界层中会出现流动转捩，在那里会出现与噪声有明显相关性的相干流动。锁相平均技术得到的压力面尾缘附近出现的相干流场是离散频率噪声的原因，在不同翼型截面、迎角和速度的情况下，相干流场是相似的。

It is well known that a discrete frequency noise is emitted even from the well-designed aerofoils for practical use in a uniform flow under some specific conditions, but the mechanism of the noise generation is not clear yet. In this research, coherent flow structures near conventional aerofoils have been investigated to study the mechanism of the noise generation as well as to obtain useful informations for its control. The following three coherent flow structures are considered : (a)a coherent flow due to transition from the laminar to the turbulent in a boundary layer, (2)a coherent flow due to a local separation on an aerofoil surface, (3)a large scaled turbulent coherent flow structure formed in the wake region of an aerofoil. The velocity fields around and behind test aerofoils were obtained and analyzed with FFT. They were also analyzed with phase-locked averaging technique using the reference signal made from the noise itself. The procedure allows us to identify the coherent flow which causes the discrete frequency noise. The results obtained can be summarized as follows ;1.Any flow separation or any coherent wake flow is not formed for the aerofoil from which discrete frequency noise is emitted. Then, a coherent flow due to a local separation and a coherent wake flow are not the cause of the discrete frequency noise.2.When a discrete frequency noise is emitted, a flow transition appears in the baoudary layer on the pressure surface near the trailing edge of an aerofoil and a coherent flow with clear correlation to the noise occurs there.3.Coherent flow fields appearing near the trailing edge on the pressure surface obtained with the phase-locked averaging technique, which are the cause of a discrete, frequency noise, are similar for many cases with different aerofoil sections, angles of attack and velocities.