An investigation of airfoil tonal noise at different Reynolds numbers and angles of attack

This paper presents the results of an experimental study into the aeroacoustics of airfoil instability noise and a linear stability analysis of the Tollmien–Schlichting (T–S) waves which develop on the pressure surface. The far field noise produced by a NACA0012 airfoil was measured in a low-noise and low-turbulence open jet wind tunnel. Noise measurements were made at the three angles of attack: 0°, 1.4° and 4.2°, and covered a range of Reynolds numbers between 1.0 × 105 and 6.0 × 105. An improved understanding of the causal link between the radiated tonal noise and the hydrodynamic instabilities which develop on the pressure surface of an airfoil for a wide range of Reynolds numbers and angles of attack is reported in this paper. The tonal noise produced by the boundary layer which develop on the suction surface was found to be negligible. It was also observed that the total T–S wave amplification on the pressure surface was generally in good agreement with the measured noise spectra. Based on the experimental and predicted data presented in this paper, it was found that effective tone noise generation requires the incoming T–S waves to be amplified by a separated boundary layer first. As the angle of attack increases, the change in pressure gradient on the pressure surface causes the instability noise to change from a broadband hump of moderate intensity level to become more tonal of higher noise intensity.