Application of Finlets for Trailing Edge Noise Reduction of a NACA 0012 Airfoil

Abstract

The present experiments investigate the reduction of trailing edge noise of a symmetric NACA 0012 airfoil using surface treatments, known as finlets. Treatment effectiveness is measured with observations from far-field data. Moreover, the highly instrumented airfoil model allows measurements of both static and dynamic surface pressure at various chord-and spanwise locations. In particular, measurements were carried out in between of the finlets, in order to elucidate clearly the near-field dynamics. With this, key parameters associated with trailing edge noise reduction could be identified. Relevant factors are, for instance, the spacing and the height of the finlets, as well as their relative positions with reference to the trailing edge. The results suggest that there possibly exists a strong correlation between the finlet height and the boundary layer thickness at the trailing edge. Attempting to identify different noise reduction mechanisms described in the previous studies, it was concluded that the prevailing one for airfoils is likely to be the detachment of small-scale turbulence structures from the airfoil surface. From the results of a position study it was inferred that shifting a treatment upstream from the airfoil trailing edge leads to beneficial effects in terms of trailing edge noise reduction compared to the configuration with the treatment applied flush with the trailing edge.

Turbulence Characteristics of Finlet Treatments Applied for Trailing Edge Noise Reduction of a NACA 0012 Airfoil

“…at higher frequencies). Gstrein et al [18] related their observation with a detachment of turbulence structures from the airfoil surface and an emerging shear layer on top of the finlet treatments, as described by Afshari et al [20].…”

“…These are mainly inspired by silently flying owl species, which have various features on their bodies and wings that contribute to the attenuation of the aerodynamic noise during their flight [10]. For a reduction of trailing edge noise, the most important approaches are trailing edge serrations [11][12][13], trailing edge brushes [14], porous materials [15][16][17] and finlet treatments [18][19][20][21][22].…”

“…They identified a reduction of surface pressure fluctuations at the trailing edge at frequencies above 2000 Hz as well as an increase at frequencies below and associated these observations with a lifting effect, moving turbulent eddies away from the airfoil surface as they are convected past the trailing edge. An attempt to identify and associate both, the channeling and dissipation effect described by Afshari et al [20] and the lifting effect defined by Bodling and Sharma [22], was performed by Gstrein et al [18]. They applied finlet treatments upstream of the trailing edge of a NACA 0012 airfoil and experimentally investigated the modifications to the boundary layer due to the presence of finlets.…”

“…In the previous study on finlet surface treatments applied on a NACA 0012 airfoil, Gstrein et al [18] established finlet parameters optimal for the reduction of trailing edge noise. The right choice of the parameter set was found to be dependent on the boundary layer thickness near the trailing edge of the untreated airfoil.…”

“…From the results of the static surface pressure, unsteady surface pressure fluctuations and velocity fluctuations in the boundary layer, they further observed a detachment of turbulence structures from the airfoil surface, which was identified as the main contributor to the reduction of trailing edge noise. Using experimental data describing the velocity auto-correlation and pressure-velocity cross-correlation, the present study focuses on the development of turbulence structures when a particular finlet treatment is applied on the NACA 0012 airfoil, which has been found to successfully reduce the trailing edge noise [18]. In doing so, the aim is to demonstrate how the varying distance of the convected turbulence structure from the airfoil surface develops along the chord and thus to provide a better idea on the actual physics responsible for the reduction of trailing edge noise.…”