An Experimental Investigation of Airfoil Tonal Noise Caused by an Acoustic Feedback Loop

Airfoils in low-to-moderate Reynolds number flows produce discrete tones which can be annoying to the human ear and potentially impede the design of fans, compressors, helicopter rotors and unmanned air vehicles. This paper discusses an experimental investigation into the generation of tones from a NACA 0012 airfoil for varying angles of attack and Reynolds numbers between 50,000 and 150,000. The investigaton employed acoustic beamforming, hotwire anemometry, single microphone measurements and surface flow visualisation techniques. The experimental results were used to calculate flow and noise parameters that were used in an acoustic feedback loop model to determine its validity. Surface flow visualisation techniques revealed locations of boundary layer separation. The phase difference between the noise signal and local flow velocities near the airfoil surface was used to measure the convective velocity of the disturbances in the airfoil boundary layer. A good agreement between the experiment and predicted tonal frequencies was obtained when the experimentally determined length and velocity scales were used in the feedback model, supporting the applicability of a feedback model for tonal noise in this case.