喷口自由剪切层自激振荡优势频率的预测

Prediction of Dominant Frequency of Self-Sustaining Oscillation Derived from Free Shear Layer

研究了喷口边缘自由剪切层在下游扰流物干涉下产生自激振荡的机理 ,并进行了相应的实验。基于对空腔流自由剪切层自激振荡各阶模态的频率方程的合理改进 ,使之能预测喷口边缘自由剪切层在下游扰流物干涉下所致的自激振荡的模态频率。实验结果表明 ,采用改进后的空腔流自由剪切层自激振荡的频率方程能较好地预测喷口自由剪切层自激振荡所致噪声的优势频率 ,并能用于工程计算。另外 ,作者在考虑声与流动耦合的复杂性时提出 :声压与涡脱落相互作用的力阻抗所致的时延 2 πα中的 α与当地马赫数有关 ,而并非为常数 0 .2 5。声的传播是弹性介质中疏密波的传播 ,脱落涡的传播是流动介质中涡质量的传播 ,声与流动耦合的力阻抗的抗性取决于它们两者在耦合时所占有的主次地位 ,由实验数据表明 ,当 Ma∞ <0 .3 4时 ,力阻抗的抗性会由质量抗转变为弹性抗 ,即 α会由正值变为负值。

The mechanism of the self-sustaining pressure oscillation derived from the free shear layer between jet edge and downstream disturbing objects is investigated and some experiments are made. By the developed frequency equation for predicting the frequency of fluid induced self-sustaining pressure oscillation in cavities, dominant frequency of the self-sustaining oscillation derived from the free shear layer between jet edge and downstream disturbing object can be predicted. Experimental results show that the accuracy of the outcome predicted by the developed frequency equation is better and they can be used for engineering calculation. Besides, considering the complexity of acoustical and flow coupling, this paper points out that the ' α ' in time delay(2π α ) is not a constant 0 25 and an associate to local Mach number. The time delay(2π α ) is the phase of the mechanical impedance from interaction between sound pressure and the shed vortex. It is worthy to emphasize that sound wave is caused by looseness and density wave in elastic medium and the transmission of the shed vortex is a transmission of vortex mass in flow medium, so that the reactance quality of the mechanical impedance is decided by superior in of interaction between the sound wave and the vortex mass. Experiment shows that mass reactance is converted into an elastic reactance when Ma ∞<0 34, it means that the value of ' α ' turns positive into negative as well.