Study on Generation Mechanism of Aerodynamic Sound

本研究的目的是通过实验研究流场和声场之间的关系来阐明气动声产生的机理。通过在轴对称射流剪切层中插入平板来产生声。结果表明：（1）声的大小与气流脉动的大小无关，而与空间速度梯度的平均值，即涡量的大小有关。当该值大于0.4时，从剪切层内部产生大的声音。2.剪切层内速度脉动的空间分布，特别是速度脉动的法向分量，与所产生的声有一定的关系。当产生大的声音时，波动围绕板边缘按顺序分布。3.低速射流的频谱特征明显，有一个频率峰值，而高速射流的频谱呈平坦分布。除了这些结果，我们还发展了一种新的方法来预测目前的声音使用边界元法。结果表明，声源是剪切层涡环通过平板边缘时产生的。

The objective of the present study is to elucidate the mechanism of aerodynamic sound generation by experimentally examining the relation betwneen the flow field and the sound field. The sound was made to generate by inserting a flat plate into the shear layr of axisymmetric jet. The results obtained are itemized as follows ; 1.The magnitude of generated sound is not related with the magnitude of flow fluctuation, but with the averaged value of spatial velocity gradient, that is, the strength of vorticity. When this value becomcs about 0.4, a large sound is generated from inside the shear layr. 2.The spatial distribution of velocity fluctuation inside the shear layr, specifically the normal component of velocity flnctuation has some relation with generated sound. The fluctuation is distributed in order around the plate edge when a large sound is generated. 3.The low speed jet has a distinguished sound with a frequency peak, while the high speed jet has flatly distributed frequenct spectrum. In addition to these results, we had developed a new method to predict the present sound by using the boundary element method. As a result, we found that the sound source is made when a vortex ring of the shear layr passes over the plate edge.