UC Davis

Airfoil self-noise results from the interaction between an airfoil and turbulence generated in its boundary layer and near wake. One of the important mechanisms of airfoil self-noise is trailing-edge noise generated by the scattering of wall pressure fluctuations beneath a turbulent boundary layer by a sharp trailing edge. It plays a critical role in many applications, such as emerging urban air mobility, airframe noise, and underwater rotors. In particular, trailing-edge noise is a dominant noise source in wind turbines, which prevents a widespread application of wind energy due to strict noise regulations and public acceptance issues. The current research leverages computational fluid dynamics to investigate novel trailing-edge noise reduction solutions, including finlets, a serration-finlet configuration with or without surface roughness, a porous trailing edge, and a porous wavy trailing edge. Not only are the aeroacoustic and aerodynamic performances of each candidate investigated but also their physical mechanism of noise reduction.

To begin with, the finlet is a bio-inspired concept from owls which are known for their silent flight. Numerical studies of the finlet are performed using an efficient approach based on Reynolds-averaged Navier–Stokes. The finlet is found to be able to reduce far-field noise up to 10 dB. Additionally, a comprehensive parameter sensitivity study is conducted to give insight into the design of finlets under different flow conditions, including spacing, height, thickness, length, and position. The physical mechanism of noise reduction with finlets is found by observing flow field results. It is shown that reduced turbulence kinetic energy near the wall and velocity deficits result in noise reduction.

The finlet study inspires two new ideas: a serration-finlet configuration with or without surface roughness. While serrations are able to reduce low-frequency noise according to literature, previous results indicate that finlets can reduce noise at high frequencies. Therefore, it is possible that noise can be reduced at all frequencies with a combination of finlets and serrations. In addition, the finlet study also indicates that the near-wall velocity deficit within a boundary layer downstream of finlets is found to contribute to noise reduction, and a greater velocity deficit usually implies more trailing-edge noise reduction. A serration-finlet configuration with additional surface roughness can further retard the flow within finlet channels, thus, result in even greater noise reduction without complicating the geometry.

Serration-finlet configurations with or without increased surface roughness are simulated using wall-modeled large-eddy simulation and improved delayed detached eddy simulation, respectively. It is found that a smooth serration-finlet configuration reduces noise by improving the hydrodynamic field at source locations, particularly by reducing near-wall velocity and turbulence kinetic energy. Quantitatively speaking, the overall sound pressure level is reduced by 20.2 dB with an improved aerodynamic efficiency. The interactions between serrations and finlets are proven to be beneficial from both aerodynamic and aeroacoustic perspectives. Moreover, additional 9 dB of noise reduction can be obtained with appropriate surface roughness.

In addition to serrations and finlets, a porous trailing edge is another promising method to reduce trailing-edge noise. Unfortunately, its flow physics of noise reduction is not well understood. Improved delayed detached eddy simulation is used again to perform a comprehensive parameter study in terms of position for porous trailing edges. Two key parameters of a porous material are porosity and resistivity. If selected appropriately, maximum noise reduction of 11.9 dB would be achieved. Instead of changing the hydrodynamic field like the serration-finlet configuration, two unique noise reduction mechanisms are identified for porous trailing edges: (1) Destructive interference is generated by distributed noise sources at multiple locations within a porous region; (2) A porous trailing edge is a less efficient noise radiator because of a smooth transition between the trailing edge and surrounding medium in terms of acoustic impedance.

The last trailing-edge treatment that this dissertation investigates is a porous wavy trailing edge which is computed using improved delayed detached eddy simulation as well. Similar to the serration-finlet configuration, this novel device is proposed based on observations on stand-alone porous and wavy trailing edges in literature. It is revealed that a porous wavy trailing edge reduces the overall sound pressure level by 8.1 dB without compromising the aerodynamic performance. The remarkable noise reduction can be mainly attributed to four major factors: (1) a significant velocity deficit upstream to the trailing edge; (2) reduced TKE in the near wake; (3) a greater distance between turbulent eddies and the sharp trailing edge; (4) breakdown of large, coherent eddies into small turbulent structures.

Overall, the current research uses both low-fidelity and high-fidelity computational fluid dynamics (or multi-fidelity computational fluid dynamics) to study novel trailing-edge treatments. It not only unravels detailed flow physics of noise reduction with these novel trailing-edge devices but also provides potential solutions to advanced low-noise aircraft and quiet wind turbines.