A Noise Reduction Study on Flow-Permeable Trailing-Edges

Abstract

This paper summarizes the results of an experimental study on flow-permeable comb-type trailing-edges which have the potential to effectively reduce turbulent boundary-layer trailing-edge noise. Basic design rules are presented with respect to a future use of such devices as suitable add-on solutions for a wide range of technical applications. These are e.g. the high-lift devices of current passenger aircraft, the blades of cooling fans or wind turbines which represent strong trailing-edge noise radiators. The noise reduction potential of several trailing-edge modifications, applied on a flat plate and on a two-dimensional NACA0012-like airfoil, was measured by a directional microphone in the open-jet anechoic Aeroacoustic Wind Tunnel Braunschweig. Additional knowledge of the incident boundary-layer flow properties was gained by application of hot-wire anemometry and unsteady surface pressure measurements. Within the range of parameters tested so far, comb design optimization appears mainly determined by geometric details of the comb itself than being dependent on specifics of the incident flow. Comb devices show the same (velocity)^5-scaling behaviour of farfield trailing-edge noise intensity as solid edges, however, on a significantly reduced level (-2 to -10 dB), and based on a constant scaling length. A minimum device length (in the order of the turbulent boundary-layer thickness or more) and a narrow slit width (of about the viscous sublayer thickness) were identified as the major design requirements. Flexibility of the comb material was found beneficial, but not essential to achieve a noise reduction.