Investigation of Airfoil Noise Generation Mechanisms at Low-to-Moderate Reynolds Number and Low Angles of Attack

Abstract

Airfoils in low-to-moderate Reynolds number flows produce a series of tones that can be annoying to the human ear and potentially impede the design of fans, compressors, helicopter rotors and unmanned air vehicles. Despite extensive studies, there is still little consensus between the various proposed noise generation mechanisms. A majority of previous studies provided evidence for existence for an acoustic feedback mechanism that was responsible for the generation of tones. Disparity exists between studies regarding the lengths of the feedback paths and the acoustic processes that produce these tones. Some studies also assert that an acoustic feedback mechanism does not exist. This thesis describes a series of experiments which were performed in a smallscale anechoic wind tunnel to investigate airfoil tonal noise generating mechanisms, using a NACA 0012 airfoil at angles of attack of 0°, 1.58° and 3.16° and Reynolds numbers 50,000 to 150,000. Experiments included single microphone measurements, acoustic beamforming, hot-wire anemometry and surface flow visualisation techniques. Single microphone measurements revealed that a strong primary tone was present with several weaker secondary tones, superimposed on a broadband spectrum. Acoustic beamforming was used to locate the acoustic source at the trailing edge. Hot-wire anemometry was used to characterise the flow properties near the airfoil surface, such as points of maximum velocity, boundary layer heights and wake velocity profiles. The phase differences between the acoustic signals and velocity fluctuations near the airfoil surface were used to measure the convective velocity of disturbances in the boundary layer. Points of instability and phase change in the boundary layer were also investigated using hot-wire anemometry. Surface flow visualisation tests revealed locations of boundary layer separation and reattachment. A dual acoustic feedback model was proposed, where feedback processes acted independently on the airfoil pressure and suction surfaces. A model proposed by Arbey and Bataille (1983) was adapted such that the feedback lengths were between the acoustic source at the trailing edge and the points of boundary layer separation on the pressure and suction surfaces. At non-zero angles of attack, acoustic feedback mechanisms existed on the airfoil suction and pressure sides simultaneously. For each Reynolds number and angle of attack, both surfaces generated tones within a feedback loop. The tones that were closely matched on both surfaces produced an acoustic superposition. Tones were generated on both airfoil surfaces independently and the tones with the same or similar frequencies constructively interfered, thus explaining the strong primary tone magnitude. The primary tone possessed near exact frequencies on both surfaces, whereas the secondary tones had weak or large differences in frequencies between both surfaces, causing little or no superposition, and thus explaining their diminished magnitude relative to the primary. Using this model, good agreement was achieved between the experimentally obtained and predicted tone frequencies.