The aeroacoustic characteristics of an oscillating wing with an NACA 0012 cross section are studied in these research by experimental means. All measurements were conducted in the 0.4m UNITED facility in Hong Kong University of Science and Technology, and the Reynolds number tested in this project varied from 0.66 to 3.3 × 10⁵. A low-noise test rig was designed and built, using which the effects of the pitching and heaving motions are studied separately. For the acoustic measurements, free-field microphones were used to measure the far-field noise spectra, and a microphone array was utilized to image the acoustic source pattern. To study the aerodynamic mechanisms that lead to the acoustic effects of the oscillating motions, the flow structures near the trailing edge were exami...[ Read more ]

The aeroacoustic characteristics of an oscillating wing with an NACA 0012 cross section are studied in these research by experimental means. All measurements were conducted in the 0.4m UNITED facility in Hong Kong University of Science and Technology, and the Reynolds number tested in this project varied from 0.66 to 3.3 × 10⁵. A low-noise test rig was designed and built, using which the effects of the pitching and heaving motions are studied separately. For the acoustic measurements, free-field microphones were used to measure the far-field noise spectra, and a microphone array was utilized to image the acoustic source pattern. To study the aerodynamic mechanisms that lead to the acoustic effects of the oscillating motions, the flow structures near the trailing edge were examined using the particle image velocimetry and hot-wire anemometry.

The characteristics of the trailing-edge noise from a static wing are firstly discussed. They provide reference for the measurement results of the oscillating cases, which can be used to demonstrate the effects of oscillation by comparison. On the other hand, the mechanisms of the trailing-edge noise from a static wing are used to explain the acoustic features of the oscillating wing. In addition, a two-cell acoustic source pattern at the harmonic frequencies on the static wing is presented and discussed.

The results obtained from the pitching and heaving wings are demonstrated separately in two chapters. The analysis of these two oscillating modes follows similar approaches. Comparing with the static cases, the oscillating motion lowers the sound pressure levels (SPLs) of the primary tones in the time-averaged spectra and increases the SPL of the broadband noise. The time-dependence of the far-field noise level and acoustic source intensity are obtained using the short-time Fourier transform (STFT) and a wavelet based time-varying beamforming algorithm, respectively. The spectrograms obtained by STFT show an alternative appearance of the tonal component in one oscillating cycle. The tones are only observed when the wing passes over a certain range of the oscillating phase position, at which moments the large-scaled coherent structures caused by the laminar boundary layer separation can be detected near the trailing edge. Correspondingly, in one oscillating cycle, the variations of the acoustic source intensity at the harmonic frequencies follow the variation trend of the tonal noise sound pressure level shown in the far-field spectrograms.