On the reductions of airfoil broadband noise through circular dimples

Abstract

This paper presents systematic and comprehensive experimental investigations into the use of circular dimples to reduce broadband noise on airfoils. The dimples are introduced on the suction surface of symmetric NACA airfoils with different thickness ratios for the control of broadband noise over a wide range of frequencies. The effects of dimple arrangements (inline and staggered) and their locations (1/3rd and 1/5th of the chord from the leading edge) on the noise characteristics are studied in detail to determine the best dimple arrangement, location, and airfoil geometry for low noise. Results indicate that the inline arrangement of dimples positioned at 1/3rdof the chord from the leading edge provides significant noise reductions of about 6–8 dB, particularly from mid to high frequencies. When the dimples are shifted to 1/5thof the chord from the leading edge, the degree of noise reduction decreases, which indicates that the noise reduction performance of airfoils decreases when the dimples are introduced near the leading edge. Further, it reveals that the inline arrangement of dimples on thicker airfoils provides superior noise reductions (6–8 dB) as compared to thinner airfoils (4–6 dB). Flow visualization reveals that the flow field is inconsistent throughout the span of the dimpled surface due to the evolution of small vortices from the dimple corner. It enhances mixing and accelerates flow structures passing over the dimpled surface, which is indicated by the presence of a high degree of spanwise decoherence/phase lag in the hotwire measurements. The presence of higher spanwise decoherence/phase lag due to the presence of the inconsistent flow field leads to significant reductions of far-field noise in dimpled airfoils. Further, the inline arrangement of dimples exhibits much higher levels of noise reduction as compared to staggered ones, due to the presence of a greater degree of spanwise decoherence/phase lag resulting from the substantial disruption of the flow field in the spanwise direction.