Aeroacoustic Performance of Fractal Spoilers

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 European Union's Optimisation for Low Environmental Noise Impact Project, looks at reducing spoiler noise while maintaining aerodynamic performance, through means of large-scale fractal porosity. It is hypothesized that the highly turbulent flow generated by fractal grids from the way the multiple-length scales are organized in space, would reduce the impact of the recirculation 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 is 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 deg. Force, acoustic, and flow visualization experiments on a flat plate were carried out and it was found that the present fractal spoilers reduce the low-frequency noise by 2.5 dB. 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 aeroacoustic study on a three-element wing system, where it was found that the fractal spoilers had a reduction of up to 4 dB in the sound pressure level, while maintaining similar aerodynamic performances as conventional solid spoilers on the measured wing system.