Broadband Noise Reduction Of Linear Cascades With Trailing Edge Serrations

Abstract

This study aims at investigating the noise reduction potential of trailing edge serrations in a linear cascade of seven loaded airfoils for $Re= 5.5 × 10^5$ and $M= 0.23$ experimentally. Turbulent boundary layer trailing edge noise is considerably reduced in the low- to mid frequency range similarly to previous single airfoil investigation. A high frequency noise increase is also found for $St=f\delta /U > 1.2$. The cascade effect is not found to deteriorate nor increase the noise reduction of serrations, except in the very low frequency range. The loading of the airfoil is not affected by the serrations but the drag is increased by $14%$. High speed particle image velocimetry is carried out in the near wake of a serration tooth and compared to the straight edge. The suction side turbulent boundary layer is removed from the airfoil surface, which could partly explained the observed noise reduction. The pressure side boundary layer crosses the serration throughs. A third turbulent region emanating from the very trailing edge is identified as a consequence of the Kutta condition. The elongated geometry of the sawteeth make it vanish faster with the downstream distance than in the straight edge case