Humans have been using wind energy for thousands of years to pump water and grind grain, and today as a renewable source of electricity at a relatively low cost. However, noise pollution is frequently cited as a major drawback. Airfoil blade self-noise is the major contributor to wind turbine noise, so reducing it is crucial in curtailing wind turbine noise overall. A new paper in the Journal of Aerospace Engineering explores the aeroacoustic characteristics of the National Advisory Committee for Aeronautics’ NACA 15-506 airfoil, similar in shape to an outer-radius wind turbine blade section, to understand the exact noise generation mechanisms and effect of airfoil stalling behavior on the generated noise.
In their paper, “Aeroacoustic Investigation of Thin-Airfoil Stall with a View to Improving the BPM Model,” authors J.A. Branch, B. Zang, D. Jones, M. Fernandino Westin, N. Bown, and M. Azarpeyvand explored the relationship among airflow around the airfoil blade and acoustics. Their data show that the airfoil can exhibit elevated low-frequency noise, typically associated with stalled flow, at angles of attack prior to stall. While current models use the AoA of stall to predict when self-noise will occur, the authors conclude that the AoA at the onset of hydrodynamic unsteadiness is a better tool to apply toward efforts at noise reduction. This research could be applied to other airfoils without the need for a full aeroacoustic investigation. Learn more at https://ascelibrary.org/doi/10.1061/JAEEEZ.ASENG-5705. The abstract is below.
Abstract
This paper presents an in-depth investigation into the near-field pressure and far-field acoustic characteristics of a National Advisory Committee for Aeronautics (NACA) 16-506 airfoil across the prestall, stall, and poststall flow regimes at a Reynolds number of 270,000. The paper specifically focuses on examining the effect of the airfoil’s stalling behavior on its self-noise. The NACA 16-506 airfoil was tested in the aeroacoustic wind tunnel facility at the University of Bristol. Remote sensors were employed to record static pressure and unsteady pressure fluctuation data on the airfoil surface. Additionally, a 78-microphone beamforming array was used to measure the far-field sound, and acoustic spectra subsequently were extracted using a delay-and-sum beamforming technique. The NACA 16-506 airfoil was found to stall by the well-established thin-airfoil mechanism, characterized by the development and growth of a leading-edge separation bubble over a range of angles of attack before bursting and leading to full-chord separation. It was observed that the near-field unsteady pressure field began to change significantly at the onset of the leading-edge separation bubble, whereas the near-field steady pressure field, and consequently the aerodynamic performance, only showed significant changes when the separation bubble burst at a higher angle of attack. It was found that the far-field acoustics changed in concert with the near-field unsteady pressure field, rather than with the aerodynamic performance of the airfoil. In essence, the airfoil self-noise was found to increase in the manner typically associated with stall at an angle of attack significantly earlier than the aerodynamic stall angle. A comparison of the measured acoustic spectra and the trailing-edge noise model of Amiet with two forms of the Brooks, Pope, and Marcolini (BPM) noise model revealed that better noise prediction was achieved when the stall switch within the model was set to the angle of attack corresponding to the onset of the leading-edge separation bubble, rather than to the later aerodynamic stall angle. This simple modification to the BPM model is useful for any airfoil known to stall via a thin-airfoil mechanism.
Learn how these findings can apply to ways to make wind turbines quieter in the ASCE Library: https://ascelibrary.org/doi/10.1061/JAEEEZ.ASENG-5705.
