An Experimental Study of Tonal Noise Mechanism of Laminar Airfoils

This experimental works summarize the (1), aeroacoustic results of airfoil self-noise generated at Reynolds numbers between 1.0 x 105 and 6.0 x 105. The test facility is a newly built low noise and low flow turbulence open jet wind tunnel, where a NACA0012 airfoil was mounted to the exit nozzle horizontally. The noise measurements were performed at three effective angles of attack at 0.0, 1.4 and 4.2 degrees. In most cases, multiple discrete tones, embedded in a broad spectrum “hump”, were recorded. The boundary layers on the pressure side of the airfoil were mainly laminar in nature, although instability waves were predicted to grow at the adverse pressure gradient region of the airfoil, (2), aerodynamic data on the airfoil wake development in an attempt to study the noise source. The airfoil incidences were chosen as 0, 2 and 5 degrees. It was found that the radiated tones from the airfoil were always accompanied by localized large scale wake oscillation. The acoustic radiation of this wake tone is believed to be triggered by resonance/feedback between the wake oscillation and the point of first instability on the pressure side of the airfoil. Linear stability analysis on the boundary layer confirmed that the frequency at which maximum amplification of the Tollmien-Schlichting wave occurred always coincide with the main tone noise frequency. It is hoped that the results presented in this paper can shed some lights on the tone generation mechanism that characterise the airfoil noise with laminar boundary layers