Active Change of Permeable Material Properties for Low-Noise Trailing Edge Applications

Abstract

For many aerospace applications, the dominant airfoil self-noise source is Turbulent Boundary Layer Trailing Edge (TBL-TE) noise. The replacement of solid airfoil trailing edges with permeable materials proved to be an adequate noise reduction mean in previous studies. This thesis project contributes to the development of innovative permeable materials by assessing the feasibility of actively influencing their noise mitigation characteristics. The proposed activation mechanism consists of heating up a polymerically coated, porous trailing edge and thereby influencing the material geometry and seepage fluid properties. Heating of the polymeric coating layer led to slightly decreasing pore diameters due to thermal expansion. The main limitation of actively changing the porous material geometry on a pore-scale level was the restriction of the coating layer thickness. Heating effects on the seepage flow through porous metal foams were analyzed experimentally. Characteristic material parameters, namely flow resistivity, permeability and form coefficient, were measured for varying fluid temperatures and it was shown that for low seepage velocities, the pressure communication across the material was negatively affected upon heating. The feasibility of actively changing far-field noise characteristics was demonstrated based on acoustic measurements in an anechoic wind tunnel. Increasing sound pressure levels were observed for both, coated and uncoated porous trailing edges upon heating. It is concluded that the dominant activation effect was the reduced communication across the porous trailing edge due to an increase in fluid temperature. The expansion of polymeric coating was not sufficient to alter the noise mitigation behavior of the porous trailing edge. Furthermore, no interactions between turbulent boundary layer wall pressure fluctuations and the soft coating layer were observed. However, it was shown that temperature control of the porous material offers a possibility to actively influence flow resistivity without modifying the geometrical structure.