An Investigation of the Genaration Mechanism of Discrete Frequency Noise Relating to the Large Scaled Turbulent Organized Structures and Its Noise Control

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Flow fields around a two-dimensional flat plate aerofoil emitting discrete frequency noise in a uniform flow have been determined, and the generation mechanism of the noise has been studied.Frequency analysis as well as phase-locked averaging technique were used for determination of flow fields. The wake velocity fields were obtained with I-type and X-type hotwire probes and the fluctuating pressure on the aerofoil surface was measured with small-sized semiconductor pressure transducers mounted in the aerofoil, whereas the sound level of emitted noise were artificially modified by inserting a circular cylinder with a small diameter in the near wake region. The flow details, such as organized structures in a turbulent wake flow as well as fluctuating pressures on the aerofoil surface, have been analyzed quantitatively, from which the generating mechanism of discrete frequency noise has been studied relating to the larged scaled organized structures in a turbulent wake.Clear coherent organized structures in a turbulent wake are confirmed in every case. The sound source of discrete frequency noise are distributed in a narrow region restricted near the traihing edge of the aerofoil. It has been quantitatively confirmed that the sound sources are generated from the fluctuating pressures due to the fluctuating velocities on the aerofoil induced from the turbulent organized structures in the wake. The span-wise correlations of the organized structures in the turbulent wake have a dominant effect on the sound pressure level of the discrete frequency noise.The calibratrion of the sound intensity mesuring system has been made, which shows that the location of the sound source can be determined within the accuracy of 1mm in the three-dimensional space.