Influence of Separated Flow on Broadband Noise of a NACA Series Blade

The streamline blade has advantages of not only the pressure drop but the aerodynamic noise since the flow attaches to the blade surface. However, the flow has a relatively large deviation angle to the blade even if the fan is driven at the vicinity of maximum efficiency point. On such a case, the broadband noise becomes a major issue in the aerodynamic noise. The purpose of this study is to clarify the influence of the separated flow of a streamline blade on the broadband noise with sufficiently large angle of attack than angle of stall. The influence of blade thickness and angle of attack on the noise was experimented by the wind tunnel with NACA series blade. The broadband noise relates to the following overall noise characteristics; the noise level of NACA0008 is larger than the NACA0018 in the vicinity of angle of attack 10 °, the noise level of NACA0008 from 10°to 20° decreases and the level of NACA0008 from 20°to 40° increases. In the vicinity of angle of attack 10°, the sound source of the broadband noise is influenced by the frequency of vortex shedding. The flow in the angle of attack 20° separated from leading edge of the blade. The velocity fluctuation in velocity shear layer in rear side is smaller than the velocity fluctuation at the separation point of the angle of attack 10°. The velocity fluctuation formed in the wake of NACA0008 in angle of attack 40° is larger than that of 20°. In the case of angle of attack 40°, the velocity fluctuation of the shear layer by the convection velocity of vortex street increased due to increase of the shear stress in the wake