An Investigation on the near-field turbulence and radiated sound for an airfoil with trailing edge serrations

Abstract

This study concerns the mechanisms of the turbulence broadband noise reduction for an airfoil with the trailing edge serrations. An open jet wind tunnel test of a SD2030 airfoil (with 4% camber, 8.0% thickness) with and without the trailing edge serrations was comparatively investigated. The Dantec Dynamics 3D hot wire anemometry was used to measure the development and intensity of turbulence boundary layer and wake of the airfoil. The noise source identifying technique with the microphone array was used to focus on the trailing edge noise radiation. The far-field acoustics DSP and fluctuating turbulence spatiotemporal information around airfoil were compared for the airfoil with standard straight trailing-edge and the airfoil with the serrated trailing-edges. It is shown that trailing edge noise could be effectively reduced with the using of serrated trailing edge, and the magnitude and frequency range of airfoil trailing edge noise reduction are changed with the increase of main flow velocity. In the frequency range of 10 kHz, the largest noise reduction of sound pressure level is about 5 dB with the using of serrated trailing edge in the test flow speed range. The predicted results of Howe’s theoretical model are in a good agreement with the experiment results when St = ωδ/Uc ≈ 1.2. However, Howe’s theoretical model over-predicted noise reduction levels in high frequency while lowly-predicted the noise suppression in low frequency range. Near-field turbulence measurements show that serrated trailing edge could reduce turbulence strength on some directions and on some positions, and could also increase turbulence strength on other directions and on other positions. This result indicates that noise reduction mechanism with the serrated trailing edge is very complicated. Some special point around serrated trailing edge should be noted in the investigation of the noise reduction of airfoil with serrated trailing edge.