Aeroacoustic Performance of Fractal Spoilers

Abstract

One of the major environmental problems facing the aviation industry is that of aircraft noise. The work presented in this paper, done as part of the EU’s OPENAIR Project, looks at reducing spoiler noise whilst maintaining lift and drag characteristics through means of large-scale fractal porosity. It is hypothesised that the highly turbulent flow generated by fractal grids, which have multiple-length-scales, would reduce the impact of the re-circulation region and with it, the low frequency noise it generates. In its place, a higher frequency noise is introduced which is more susceptible to atmospheric attenuation and could be deemed less offensive to the human ear. A total of nine laboratory scaled spoilers were looked at, seven of which had a fractal design, one with a regular grid design and one solid for reference. The spoilers were inclined at an angle of 30 ◦ . Force, acoustic and flow visualisation experiments on a flat plate were carried out, where it was found that the present fractal spoilers reduce the low frequency noise by 2.5dB. Results show that it is possible to improve the acoustic performance by modifying a number of parameters defining the fractal spoiler, some of them very sensitively. From these experiments, two fractal spoilers were chosen for a detailed aero-acoustic study on a three-element wing system, where it was found that the fractal spoilers had a 1dB reduction in the sound pressure level and were also able to maintain the amount of lift and drag generated by the wing system.