The Development of a Research Technique for Low Speed Aeroacoustics

Abstract

The aerodynamic sound generated by wind turbines was identified as a growing concern within the industry. Prior to performing wind turbine aeroacoustic research, however, a technique suitable for studying low speed airfoils needed to be designed, serving as the primary research objective. A review of aeroacoustic theory and literature indicated that low speed flows are best studied using experimental methods, leading to the design of a near field pressure measurement technique. To facilitate the near field pressure measurements, a custom piezoelectric sensor was developed, exhibiting a pressure and frequency range of approximately 67 to 140[dB], and 100 to 10000[Hz], respectively. As a secondary research objective, a series of experiments were performed to validate the designed technique. The experiments were performed in a non-anechoic wind tunnel using a cylindrical test specimen. Using the near field pressure measurements, as well as a simple far field measurement, the sources of aerodynamic sound were effectively resolved. The Strouhal numbers corresponding to the contributing flow structures were generally within 1.5[%] of correlation based predictions. The near field pressures were consistently 10 to 15[dB] higher than the far field, quantifying the benefit of the near field technique. The method was also effective in detecting the decreasing coherence of the aeroacoustic sources with increasing Reynolds number. A minor deficiency was observed in which the ability to localize aeroacoustic sources was impeded, however, the cylinder experiments were particularly vulnerable to such a deficiency. Although the near field pressure measurements were shown to be effective in characterizing the aeroacoustic sources, a number of recommendations are presented to further improve the flexibility and measurement uncertainty of the experimental technique.