侧壁约束效应对三维方腔自持振荡和噪声辐射影响的实验研究

实验研究了侧壁约束效应对三维方腔流动结构和噪声辐射特性的影响，固定方腔长深比为2∶1，使用麦克风阵列测量了方腔宽长比从0.1变化至0.5过程中流致噪声在不同指向性下的强度变化规律，并使用脉动压力传感器测量了不同宽长比方腔内部壁面压力分布，同时采用TR-PIV（Time-Resolved Particle Image Velocimetry）测量了方腔内流动结构的发展。实验结果表明:对于宽长比为0.5的方腔，当来流马赫数大于0.03时，方腔流动开始出现振荡并向上游辐射噪声；当来流马赫数增大至0.20时，方腔流动发展为对应Rossiter三阶模态的自持振荡，并辐射出尖频噪声。减小方腔宽度，当宽长比小于0.3时，方腔流动的自持振荡和尖频噪声被大幅度抑制甚至消除，来流马赫数为0.20和0.25时，方腔上游总声压级能够降低3 dB以上。通过对比壁面压力分布和PIV流场测量结果，发现减小方腔宽长比时，方腔内主回流涡向上游移动，涡强度降低，使得方腔的流动反馈不足以形成自持振荡，从而降低了辐射噪声。

Experimental study of the effects of confinement on self-sustained oscillations and noise radiation in three-dimensional open cavities

Flow past an open cavity has been studied extensively, while less attention has been paid to the effects of confinement due to side walls, which produces rich flow dynamics and noise characteristics. In this study, the effects of confinement on flow structures and noise radiation in three-dimensional rectangular cavities are investigated experimentally. The length and depth are fixed, and five ratios of width/length （W/L=0.1–0.5） are considered. The measurements are performed in an acoustic wind tunnel. The pressure oscillations are onset after the wind speed is greater than Ma 0.03. Once the wind speed is greater than or equal to Ma 0.20, the flow and noise radiation are dominated by the self-sustained oscillations corresponding to the third Rossiter’s mode. Furthermore, the present experiments show that the local pressure oscillations and noise radiation of this frequency can be weakened or even eliminated when W/L is equal to or less than 0.3 for the wind speeds of Ma 0.20 and Ma 0.25. The upstream OASPLs in the far field can be reduced by more than 3 dB when W/L decreases from 0.4 to 0.3 at Ma 0.20. By analyzing the surface pressure and TR-PIV（Time-Resolved Particle Image Velocimetry） results, it is found that the suppression of the tonal noise is closely related with the changes of the primary recirculation and some secondary vortical structures by decreasing W/L. In particular, the intensity of the primary recirculation is greatly weakened with strong confinement effects, and the feedback process is not strong enough to produce self-sustained oscillations.