Measurement of Aerodynamic Noise and Unsteady Flow Field around a Symmetrical Airfoil
The purpose of this paper is to study the physics of aerodynamic noise generation from the symmetrical airfoil of NACA 0018 in a uniform flow. The relationship between the noise spectrum and the unsteady flow field around the airfoil is studied in an acoustic wind tunnel using flow visualization and PIV analysis. The discrete frequency noise was generated from the airfoil inclined at small angle of attack to the free stream. The flow visualization result indicates the presence of attached boundary layer over the suction side and the separated shear layer over the rear pressure side of the airfoil, when the discrete frequency noise is observed. It is found from the PIV analysis that a large magnitude of vorticity is generated periodically from the pressure side of the trailing edge and it develops into an asymmetrical vortex street in the wake of the airfoil. The periodicity of the shedding vortices was found to agree with that of the frequency of the generated noise. Keywords: aerodynamic noise, unsteady flow, visualization, PIV, airfoil. 1. Introduction It is well known that the aerodynamic noise is generated from the airfoil by the interaction of the flow with the airfoil itself in a stream. The generation of such noise is often observed in turbo-machines, such as fan, blower, windmill and so on, so the study on the aerodynamic noise is very important in relation to the noise reduction in such turbo-machines. When the airfoil is located in a stream at relatively low Reynolds numbers, the laminar boundary-layer prevails over the airfoil surface and a discrete frequency noise is generated from the airfoil inclined at small angles of attack to the free-stream (Paterson et al., 1973). The basic mechanism of this noise generation was studied by Tam (1974), who suggested that the noise could be emitted from the self-exited feedback loop consisting of large-scale unstable disturbances in the boundary layer, the wake flow and the feedback acoustic waves. Later,...