Schlieren-Based Visualization of Acoustic Fields in Lined Ducts

Acoustic liners are widely employed to reduce aircraft noise, especially engine fan noise. To achieve high efficiency with resonant liners, it is vital to first understand the mechanism of the liners under grazing flow conditions. Visualization has been useful for this purpose, although experimental attempts are limited despite their benefit. This study utilizes the schlieren method to visualize sound propagation and attenuation in the vicinity of resonant acoustic liners. A rectangular metal duct containing a test section with transparent glass windows is fabricated, and a two-dimensional slit liner is mounted flush within the duct. The air density gradients in the duct and liner under resonance conditions are visualized. The resulting visualized movies show phenomena associated with the generation and dissipation of vortices at the liner opening at a sound pressure of 152 dB, both with and without grazing flow. Detailed analysis of luminance distribution in the visualized images, including high-pass filtering and wavelet analysis, provides additional insights. The observed phenomena are qualitatively similar to the preceding numerical results. The proposed method offers an effective tool for understanding noise reduction mechanisms in liners and enables testing under a broader range of conditions.