MECHANISM OF THE TRANSFORMATION OF PRESSURE FLUCTUATION PRODUCED BY KARMAN VORTEX SHEDDING INTO NOISE

本文从理论和实验两方面研究了NACA 0008和NACA 0015叶片在均匀来流中的近尾流和旋转平板近尾流中的速度脉动特性与噪声的关系。结果表明：（1）对于NACA 0015厚叶片，当攻角θ小于5 °时，会产生离散频率噪声;在这种情况下，压力侧的流动在叶片后缘的上游分离，产生卡门涡街，这是产生离散频率噪声的原因。(2)In对于较薄叶片NACA 0008，如果θ =0，DFN不会出现，因为没有逆压梯度强烈的位置。然而，如果有一些攻角，即使它很小，由于其相对尖锐的几何形状，流动在叶片的前缘附近分离。然后产生离散频率噪声。(3)In NACA 0008叶片周期性速度波动的情况实际上是在离叶片后缘相对较远的地方产生的，然而，DFN不会发生。其原因是气流附着在叶片后缘。这一事实清楚地表明，DFN产生的必要条件是在近尾流中存在一个大的死流区。（4）虽然如旋转流场的情况一样，迎面流在翼展方向上被剪切，但会发生DFN。这意味着DFN的产生机制与均匀主流中的DFN基本相同。（5）旋转平板产生的噪声频谱的宽带特性可以通过物理模型进行理论解释，在该物理模型中，音调噪声是由每个卡门涡脱落引起的周期性速度波动局部产生的，并且它们在叶片翼展方向上被积分。

It is theoretically and experimentally investigated that the generated noise is closely related with the characteristics of the velocity fluctuation in the near wakes of a blade,NACA 0008 or NACA 0015 placed in a uniform oncoming flow,and in the near wake of a rotating flat plate. The results are summarized as follows:(1)In the case of a thicker blade,NACA 0015,the discrete frequency noise is generated if the attack angle theta is less than about 5 degree. The flow on the pressure side, separates in this case just upstream of the trailing edge of the blade and the Karman vortex street generates,which is the cause to generate the discrete frequency noise. (2)In the case of a thinner blade,NACA 0008,the DFN does not occur if theta=0,because there is no location where adverse pressure gradient is intense. However if there is some attack angle,even if it is small,the flow separates near the leading edge of the blade due to its relatively sharp geometry. Then discrete frequency noise generates. (3)In the case of NACA 0008 blade periodic velocity fluctuation is actually generated relatively far from the blade trailing edge,the DFN,however,does not occur. The reason of this is the flow attached up to the blade trailing edge. This fact shows clearly that the necessary condition of the generation of DFN is the existence of a large dead flow region in the near wake. (4)Although the oncoming flow is sheared in the span direction as in the case of the rotating flow field,DFN occurs. This means the mechanism of the generation of DFN is fundamentally the same that in the uniform main flow. (5)The broad band nature of the spectrum of noise generated from the rotating flat plate can be theoretically explained by the physical model in which the tonal noises are generated locally by the periodic velocity fluctuations due to each Karman vortex shedding and they are integrated in the blade span direction.