Experimental study on flow and noise characteristics of NACA0018 airfoil

The flow characteristics on and around an airfoil at moderate Reynolds number are studied to understand the generation mechanism of tonal noise from a symmetrical airfoil NACA0018 in a uniform flow. The separation and reattachment of the flow on the airfoil surface are evaluated from the liquid-crystal visualization and the velocity field across the boundary layers over the airfoil are measured by particle image velocimetry (PIV). These results indicate that the separation and reattachment points of laminar boundary layers over the airfoil move along the airfoil surface, depending on the angle of attack variations. When the airfoil is inclined at a small attack angle to meet with the condition of tonal noise generation, the boundary layer on the pressure surface experiences the separation along the surface and reattaches near the trailing edge of the airfoil. Then, the periodic vortex structure is generated over the pressure surface near the trailing edge, which is followed by the formation of periodic vortex shedding in the airfoil wake. With further increase in attack angle, the flow over the suction surface separates near the leading edge of the airfoil and the boundary layer on the pressure surface keeps fully attached flows on the airfoil surface, resulting in the disappearance of tonal noise.