Numerical and experimental study of flow field characteristics of the zigzag-shaped collector in a large-scale aeroacoustic wind tunnel

The Beihang University Acoustic Wind Tunnel (BHAW), with dimensions of 4 m × 3 m, is a large-scale low-turbulence, low-noise aeroacoustic wind tunnel primarily used for studying the aerodynamic noise mechanisms and noise reduction technologies of large aircraft components. During wind tunnel operation, the geometry and structural dimensions of the collector section play a crucial role in smoothly guiding the airflow, reducing airflow impact on the walls, and mitigating low-frequency pulsations and noise in the collector section. Based on a systematic analysis of the impact of test section jet on different collector configurations and their flow characteristics, this study introduces a novel zigzag-shaped collector design and optimizes its dimensions through three-dimensional numerical simulations. The final selected structural parameters for the BHAW wind tunnel's collector are as follows: the ratio of the collector's exit area to the test section's cross-sectional area is 1.352; the ratio of the length of the horizontal straight segment at the inlet to the length of the test section is approximately 10%; the ratio of the radius of the circular lip to the height of the test section exit is 0.085; and the single-side contraction angle is 8°. Numerical simulation results indicate that when the contraction angle of the collector section exceeds 8°, significant flow separation occurs, and the separation region expands progressively with increasing angle. The impingement area ratio increases approximately linearly with the angle. In contrast, the zigzag-shaped collector effectively prevents flow separation, ensuring that the jet impingement point consistently remains at the lip, thereby maintaining stable flow characteristics. Large eddy simulation (LES) results further demonstrate that the wall pressure fluctuations in the zigzag-shaped collector are relatively small. The dominant vortex structures are primarily shedding vortices from the jet boundary layer, with a characteristic frequency of 13.3 Hz and a corresponding Strouhal number (St) of 0.57. Wind tunnel measurements indicate that within the velocity range of 35–100 m/s in the closed test section, the dynamic pressure stability coefficient ranges from 0.00197 to 0.001, satisfying the advanced standard η\leqslant 0.002. In the open test section, under 35–80 m/s, the coefficient ranges from 0.0048 to 0.0036, meeting the basic standard η \leqslant 0.005. The pressure stability coefficient in the open section is approximately 2.4 to 3.6 times that of the closed section.The dynamic pressure coefficient distribution in the model region of the open test section is within ±0.5% at the design velocity, meeting the basic qualification criteria of the National Military Standard of China (GJB). For the closed test section, the distribution is within ±0.2%, meeting the advanced criteria of the same standard. At the design speed of 80 m/s, the far-field noise level of the open test section is measured to be between 74.0 and 74.4 dB(A). Compared with the RTRI wind tunnel, the BHAW tunnel achieves approximately 7 dB noise reduction in the low-frequency range, exhibits comparable noise levels in the high-frequency range, and has an overall sound pressure level that is 0.6–1.0 dB lower than that of the RTRI. In summary, the zigzag-shaped collector design is rational and effective, enabling stable flow in the test section and reducing both turbulence and noise, thereby demonstrating strong potential for practical engineering applications.