Acoustic Characterization of High-Speed Jet Using Air Tab

Abstract

Jet noise production is a complex phenomenon characterized by the distribution of noise sources along the jet. High-frequency noise components primarily emanate from regions near the nozzle exit, while low-frequency noise is generated farther downstream, around the end of the potential core. This spatial distribution necessitates measurements taken at significant distances from the noise source to accurately characterize jet noise. One method to control and study this noise involves the use of air tabs, which are active control devices placed at the exit plane of the nozzle, oriented perpendicular to the flow. In this study, the flow from a Conical Convergent-Divergent nozzle to the atmosphere under perfectly expanded conditions was simulated using the Fluent software package. Various air tab inlet pressures were tested to observe their effects on the flow and noise characteristics. Virtual receivers were strategically placed at distances of 10d, 30d, and 50d from the nozzle exit, where 'd' represents the nozzle diameter. These receivers were positioned at angles ranging from 0° to 130° in 20° increments to capture a comprehensive set of data on the jet noise. The analysis focused on different air tab inlet pressures, particularly noting the behavior at an inlet pressure of 449,234.52 Pa (1.6 times the initial pressure, Po). It was found that this specific pressure setting resulted in a more pronounced characteristic decay of the jet flow properties compared to other cases. However, when conducting a spectral analysis of the pressure at the receivers, an increase in the Sound Pressure Level (SPL) was observed. Specifically, the SPL increased by approximately 6 dB compared to a free jet, and this increase in SPL correlated with higher air tab inlet pressures. This finding indicates that while air tabs can effectively modify the flow characteristics and potentially reduce certain types of noise, they may also introduce higher noise levels under specific conditions. The use of air tabs at the studied pressure resulted in a significant change in the flow structure, which in turn affected the noise distribution and levels. The increase in SPL suggests that the control mechanism introduced additional turbulence or altered the jet dynamics in a way that amplified the noise. In summary, the study demonstrates that the integration of air tabs at the nozzle exit can influence the jet flow and noise characteristics significantly. While certain configurations may enhance thrust and modify noise profiles favorably, they can also lead to increased noise levels in other scenarios. This underscores the need for careful optimization and consideration of trade-offs when designing and implementing noise control strategies in supersonic jets. The findings provide valuable insights for future research and development aimed at mitigating jet noise while maintaining or improving aerodynamic performance.