Airfoil dual acoustic feedback mechanisms at low-to-moderate reynolds number

The tonal noise production of airfoils at low-to-moderate Reynolds numbers has been under scrutiny for decades, with various propositions of vortex shedding processes and acoustic feedback mechanisms. This paper presents an experimental investigation performed in an anechoic wind tunnel to investigate airfoil tonal noise generating mechanisms, using a NACA 0012 airfoil at angles of attack of 0° and 1.58° and Reynolds numbers of 50,000 to 150,000. Experiments included single microphone measurements, acoustic beamforming, hot-wire anemometry and surface flow visualisation techniques. A dual acoustic feedback model is presented, where feedback processes act independently on the airfoil pressure and suction surfaces. It is proposed that tones are generated on both airfoil surfaces independently within a feedback loop at each Reynolds number and angle of attack. The tones that were closely matched on both surfaces produced an acoustic superposition, such that the tones with the same or similar frequencies constructively interfere, explaining the strong primary tone magnitude. The primary tone possessed near exact frequencies on both surfaces, whereas the secondary tones had larger differences in frequencies between both surfaces, causing little or no superposition, thus explaining their diminished magnitude relative to the primary tone. Good agreement was achieved using this model between the experimentally obtained and predicted tone frequencies.