Understanding and reduction of the aerodynamic noise emitted from surface-mounted obstacles and airfoil trailing edge

Abstract

The noise radiated by a rigid plane surface under a turbulent boundary layer flow is a matter of great importance in various fields, including aviation and wind energy. With the continuous growth in the number of civilian flights and wind turbine installations, there is a growing concern among the public regarding noise emissions. This underscores the need for comprehensive investigations into the noise generation mechanisms and the implementation of effective mitigation strategies. This dissertation focuses on the self noise generated by the surface discontinuities under turbulent boundary layer, specifically by two-dimensional obstacles mounted on a flat plate and the trailing edge of a wing model. The flow around surface-mounted obstacles exhibits greater complexity, including flow impingement, turbulence generation, the formation of separation bubbles, shear layer formation, and subsequent reattachment downstream. On the other hand, the turbulent flow over the trailing edge of a wing model is relatively simpler and may involve flow separation, turbulence generation, and vortex shedding. Although the nature of flow differ for these two cases, they both involve noise production when turbulence encounters a discontinuity. First, the turbulent boundary layer flow over surface-mounted obstacles are investigated. The study focuses on comparing the flow and noise characteristics of flow over square, triangular and semicircular obstacles mounted on a flat plate. The flow results reveal that the presence of sharp leading edges on the obstacles significantly modifies the shear layer characteristics and the reattachment lengths. For the same obstacle height, the square and triangular obstacles with sharp edges exhibit lower shedding frequencies compared to the smooth semicircular obstacle. The effect of sharp leading edge also seen in the noise characteristics. The noise radiated by the square and triangular obstacles is found to be comparable and considerably higher than the semicircular obstacle. Furthermore, the noise directivity for the sharp-edged square and triangular obstacles exhibited a streamwise dipole pattern, oriented in the upstream and downstream directions. In contrast, the smooth semicircular obstacle radiated noise equally in all polar angles. Second, the noise reduction capabilities of the trailing edge serration are highly susceptible to the flow misalignment, resulting in a significant decrease in performance, particularly under loading conditions. To address this issue, this study introduces two flow control techniques aimed at enhancing its effectiveness in such conditions. These techniques include serration extension and streamwise vanes treatment. Particle image velocimetry and hotwire measurements shows the effectiveness of these techniques in mitigating cross-flow. Among the two investigated techniques, the serration extension proved to be the most effective, resulting in noise reduction of 10 dB. Furthermore, the lift and drag performance was not affected by the serration extension and streamwise vane treatments, as indicated by load cell measurements. Finally, efforts have been made to address the issue of blunt trailing edge vortex-shedding noise by implementing two approaches: the utilization of vortex generators and streamwise vanes at the trailing edge. The objective is to disrupt the shedding of vortices by introducing either streamwise vortices into the flow or turbulent fluctuations at specific locations along the span. Results reveal that the streamwise vanes is more effective than vortex generators in reducing the vortex-shedding tonal noise. In summary, this thesis provides valuable insights into the variations in flow and noise characteristics between smooth and sharp leading edge surface-mounted obstacles. The novelty of the surface mounted obstacle study is the comparative noise analysis of various geometries and its emphasis on identifying the dominant noise source for each of these geometries. Additionally, it presents novel trailing edge noise reduction treatments that hold promise for future high-lift devices. Especially, the serration extension showed excellent noise reduction and aerodynamics performance in the wind tunnel tests on a symmetric airfoil. Subsequently, its practical applicability can be explored through testing on real wind turbine blades in the future.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Xin ZHANG (Supervisor) & Siyang Zhong (Supervisor)