Porous Airfoils: Noise Reduction and Boundary Layer Effects

The present paper describes acoustic and hot–wire measurements that were done in the aeroacoustic wind tunnel at the Brandenburg University of Technology Cottbus on various SD7003–type airfoils made of different porous (flow permeable) materials. The objective of the research is the analysis of the turbulent boundary layer properties of porous airfoils and, subsequently, of the noise generated at the trailing edge. The influence of the porous materials, characterized by their air flow resistivity, is discussed. The acoustic measurements were performed using a planar 56–channel microphone array and the boundary layer properties were measured using constant temperature anemometry. The recorded acoustic data underwent further processing by application of advanced beamforming algorithms. A noticeable reduction of the emitted trailing edge noise was measured for the porous airfoils over a large range of frequencies. At high frequencies, some of the porous airfoils were found to generate more noise than the reference airfoil which might be due to the surface roughness noise contribution. It is found that the turbulent boundary layer thickness and the boundary layer displacement thickness of the airfoils increase with decreasing flow resistivities for both suction and pressure side. Both boundary layer thickness and displacement thickness of the porous airfoils are greater than those of a non-porous reference airfoil