リングファンの動翼回転騒音に及ぼす後流の影響

概要

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Ring fan is a propeller fan that applies an axial-flow impeller with a ring-shaped shroud on the blade tip side. The fan has been expected not only to improve the aerodynamic characteristics but also to reduce the aerodynamic noise. In this study, the influence of wake on aerodynamic noise of a ring fan has been discussed by comparison with the characteristics of a current propeller fan. The blade rotating noise was assumed for the mechanism of the discrete frequency noise in the ring fan. Moreover, the theory for the estimation of the blade rotating noise based on the wake characteristics has been proposed. The overall noise level of the ring fan was decreased 3 dB than that of the propeller fan due to the discrete frequency noise in the first harmonics. As a result of CFD analysis, it is considered that the discrete frequency noise was generated from the blade surface of the impeller by the periodic pressure fluctuation synchronizing with the blade passing frequency. According with measured flow patterns, the relative velocity of the ring fan became slightly high than the propeller fan whereas the velocity fluctuation of the ring fan decreased approximately 71% than that of the propeller fan in the near wake. From results of the estimation based on the proposed theory of the aerodynamic noise, it can be considered that the discrete frequency noise of the ring fan was decreased by the perturbation of the jet-wake flow in the wake.