Reduction of Emitted Sound by Application of Porous Trailing Edges

Abstract

The reduction of trailing edge sound by application of porous material at the rear part of a NACA0012 airfoil is investigated numerically. A simple, anisotropic porosity is used, namely narrow streamwise slits. They extend by 11:25% of the chord length upstream from the trailing edge. The slits completely permeate the trailing edge from the upper side of the airfoil to the lower side. Three colluding fundamental mechanisms are suggested to be responsible for the reduction of trailing edge sound: The change of turbulence production due to the slits, the acoustic absorption, and the change of the sound generating process at the trailing edge. These mechanisms are analysed systematically by means of Computational Fluid Dynamics (CFD) and Computational Aero-Acoustics (CAA). The CFD results show no significant impact of the narrow slits on the turbulence production. Two different kinds of acoustic sources are modelled and applied to both, a porous trailing edge and a solid reference configuration. On the one hand, the acoustic absorption is investigated by use of a constantly vibrating dipole at the trailing edge which is generating the familiar directivity of trailing edge sound. On the other hand, a single advecting vortex is initialised to examine the interaction between the turbulence and the airfoil concerning the generation of sound. The current computational results show the same range of acoustic benefit of 6 dB like former wind tunnel tests. The findings illustrate that the modified sound generating process has the major influence, while pure absorption is important only in a narrow range of propagation angles.