Investigation of tonal noise generation on an airfoil with time-resolved PIV

Tonal noise generated by airfoils at low to moderate Reynolds numbers has been focus of research since decades. Several explanations of the observed phenomena relate the noise spectrum to the instability of the laminar boundary layer, which has lead to the term laminar boundary layer instability noise. The particular features of boundary layer instability noise have been reported in literature, but remain controversial to date. In the past, studies have been performed relying on hot-wire, Laser Doppler Velocimetry (LDV), phase-locked Particle Image Velocimetry (PIV), and acoustic measurements, as well as numerical simulations and theoretical approaches. In recent years, high-speed PIV has become accepted as a tool for the investigation of aeroacoustic problems since it can provide spatio-temporal information on the structure of the source field. In the present study, far-field microphone measurements are conducted to map the noise emissions of a NACA 0012 airfoil over a range of angles of attack and velocities. The principal features observed in the noise maps agree with findings of previous experiments. For the investigation of the ow structure and source field at the trailing edge, high-speed PIV in conjunction with simultaneous microphone measurements has been performed. The information obtained using this recent measurement technique provides detail on the relation between the acoustic spectrum and the dynamic characteristics of the aeroacoustic source field.