Generation mechanism of flow-induced noise of tandem-cavity systems at subsonic speeds: Fluctuation correlation

To explore the flow-induced noise generation mechanisms of tandem cavities at subsonic speeds, we conducted wind tunnel experiments to measure the near-field wall pressure fluctuations and far-field noise characteristics of a two-cavity configuration. Multiple data analysis methods were employed to investigate the flow fluctuation correlations, flow mode characteristics, far-field noise components, and their generation mechanisms. The incoming flow velocity was 70 m/s. One cavity had a fixed length of 160 mm, while the other was adjustable to 160, 240, 320, and 480 mm. The cavity spacing was set to either 80 or 160 mm. The results show that when two cavities are arranged in tandem in the flow direction, the upstream cavity serves as the primary source of narrowband noise, while the downstream cavity generates predominantly broadband noise due to the suppression of its narrowband component. The overall shape of the far-field noise spectrum is mainly determined by the total length of the tandem-cavity system, with longer lengths leading to stronger low-frequency broadband noise below 400 Hz. The narrowband noise generation mechanism of the upstream cavity primarily depends on whether the two cavities are of the same length. When the tandem cavities have different lengths, the narrowband components in the upstream cavity remain largely unaffected, with characteristic frequencies and fluctuation intensities similar to those of a single cavity. When the cavities are of equal length, changes in the cavity spacing significantly influence the narrowband noise generation mechanism of the upstream cavity. At a spacing of 80 mm, mixing of the shear layers occurs, causing a slight downward shift in the characteristic frequency. At a spacing of 160 mm, the internal flow correlation within the upstream cavity significantly increases, thereby exciting lower-order flow modes and enhancing the intensity of the dominant mode, while the characteristic frequency remains unchanged.