Trailing Edge Noise Reduction Technologies for Applications in Wind Energy

Abstract

A selection of trailing-edge modifications targeted towards trailing-edge noise reduction was tested in the Acoustic Wind Tunnel Braunschweig (AWB) of the German Aerospace Center. The measurements were focused on the transfer of known noise reduction technologies from aerospace to wind energy application. For this purpose, these technologies were applied to a NACA 64-618 airfoil and a newly developed low-noise airfoil, RoH-W-18%c37. The applied noise reduction technologies are in the form of plate extensions (1) comb-type trailing-edge: brush and slotted, (2) serrated trailing-edge, and (3) porous trailing-edge. Acoustic measurements were conducted for the geometrical angles of attack $\alpha_g$ = 4$^\circ$ , 7$^\circ$ , 11$^\circ$ and the freestream velocities U$_\infty$ = 40, 50, 60 m/s corresponding to the chord-based Reynolds numbers Re = 0.8, 1.0, 1.2$\times$106 . The low-noise airfoil, RoH-W-18%c37, shows an aerodynamic improvement while providing similar noise characteristics to the NACA 64-618. For cases with the maximum extent of the laminar boundary layer, 2 dB noise reduction at the boundary layer thickness based Strouhal number St $\approx$ 0.5 was observed without any trailing-edge modifications. Noise reductions of 7-10 dB at St < 1 were measured for trailing-edge modified with the brush extension. A noise increase for the serrated trailing-edge was observed at St > 1. This was minimized with the use of the porous serration with the detriment of the noise reduction at St < 1. The use of microperforated trailing-edge extensions leads to 1 - 4 dB noise reduction and their noise reducing attribute is not as consistent as for the other two types of extensions.