Numerical Investigations of Bio-Inspired Blade Designs to Reduce Broadband Noise in Aircraft Engines and Wind Turbines

Abstract

The unprecedented growth in the use of unmanned aerial vehicles (UAVs), civil aviation, and wind turbines gives a new dimension to the problem of noise emitted by these machines the noise problem will no longer be limited to spatial pockets near airports or wind farms. UAVs will be flying a few hundred feet over our houses and landing in our front yards. Wind turbines similarly will be in much closer proximity to our homes with the five-fold increase in wind energy capacity envisioned by the Department of Energy. The increased number and proximity of these machines to humans is bound to exacerbate the aerodynamic noise issue. The adverse effects on health of noise in general, aircraft noise, and wind turbine noise, are well known. With such widespread impact, noise reduction is no longer a nice-to-have, but an enabling technology for next-generation energy and propulsion machines. Efforts to silence these machines will undoubtedly have a significant impact on our lives. Nature inspires great innovations. Bio-mimicry has already provided some outstanding innovations in a variety of engineering applications. Termite mounds have inspired temperature-regulated buildings, the hydrophobic lotus leaf has inspired self-cleaning paints, the kingfisher bird has inspired Japan’s Shinkasen (high-speed train), etc. One such biological marvel that is yet to find its due application in engineering is the night owl with its ability to fly silently. Of all the vehicles known to man only the owl is capable of almost silent flight. One species of the night owl the common barn owl (Tyto alba) is particularly adept at flying silently. Henceforth, the common barn owl shall be referred to as the owl. The owl has evolved for over 20 million years to perfect its silent flight. Ornithologists have reported the unusually quiet flight of the owl as early as in the 1930s. It is likely that the acoustic stealth capability (“hush kit”) of the owl has been known for much longer. The owl needs to have silent flight for two reasons: (1) to aurally locate the prey in the dark; this supplements its brilliant vision, and (2) to avoid aural detection by the prey. The owl is completely inaudible in its flight if it is more than 3 meters away. Early investigations have reported three key anatomical features (unique feather adaptations) of the owl that enable its acoustically silent flight: