Trailing-edge noise attenuation of airfoil by means of comb-serration at low Reynolds numbers

Abstract

Low Reynolds number flow is three-dimensional and intricate due to multiple vortical phenomena. This research contributes by investigating the impact of laminar Separation Bubble (LSB) on noise generated by passive control techniques. It also enhances the understanding of the efficiency of various trailing edge designs such as serrations, comb, comb-serrated, and porous configurations, across different flow conditions and Reynolds numbers, while also addressing the limitations of existing geometrical models for trailing edges. The study intends to examine the performance of different configurations, emphasizing their effect on flow structure and acoustic responses. The methodology of this study encompasses a combination of techniques that includes conducting 2D simulations using the SST model, performing 3D simulations using large eddy simulation, employing FW-H acoustic modeling, and utilizing an experimental PIV setup. These methods collectively provide a comprehensive and robust platform for in-depth exploration of the research objectives. The analysis of the NACA0015 airfoil's flow characteristics revealed the presence of laminar separation bubbles (LSBs) at low Reynolds numbers and angles of attack. Two types of flow patterns, with and without reattachment, were identified. On the suction side, Increasing the Angle of attack leads to a noticeable upstream shift of these points, while they move downstream along the pressure side. In 3D simulations, pressure distribution was symmetrical, with the maximum at the leading edge. No separation was observed except at the trailing edge tip. At higher angles of attack, the baseline airfoil experienced flow disturbances, laminar separation bubbles, and vortex shedding. The serrated, combed, and comb-serrated designs exhibited more stable flow patterns and fewer separation bubbles than the baseline, potentially reducing tonal noise. Conversely, the poro-serrated design led to distorted flow and an upstream-moving separation bubble, suggesting a possible increase in tonal noise. Moreover, results showed irregular broadband noise (300 - 600 Hz) with increased noise and shifting peak frequency as the Angle of attack rose. The serrated trailing edge design notably reduced noise levels by roughly 21 dB, especially for low frequencies. Comb-serration increased high-frequency noise by about 9 dB for angles of attack at 0, -1, and -2 degrees reduced approximately 9 dB for angles of attack at 1 degree and 2 degrees. On the other hand, the directivity pattern showed that the maximum noise level is observed to predominantly radiate at an azimuth angle of around 90 degrees for all the cases, ranging from 90 to 270 degrees, indicating that the majority of the source's acoustic energy is being emitted on the suction and pressure sides of the wing. In conclusion, the findings demonstrate that serrated and comb-serrated designs are beneficial in reducing noise levels, and that the Angle of attack can significantly impact both the noise level and directivity pattern.