Morphology, Flight Kinematics And Bionics Of Silent Flight Owl

With the development of economic and social, the noise has become the third pollutionexcept for air and water pollution. The aerodynamic noise has a large proportion of noisepollution. Reducing the aerodynamic noise and environmental pollution is becoming thefront of scientific and technological research.Many species of owls, including tawny owl(Strix aluco), barn owl(Tyto alba) and theeagle owl(Buto Bubo), have the ability to fly silently. Its prey, typically rodent animal, hasacute hearing which can make up for the terrible vision at night. However, the quiet owl inboth gliding and flapping flight generates noise at low frequencies below2kHz, which isbelow its prey's hearing range(2-20kHz). Hence, the owl is almost flight silent to its prey.The silent flight of owl has given a new idea and method for noise reduction engineering andhas inspired the engineer to reduce the aerodynamic noise.Based on the bionics theory, the study was focused on the silent flight owl, includingthe eagle owl(Bubo bubo) and long eared owl(Asio otus). The aeroacoustic, morphology andgeometry of feather, kinematics characteristic of flight of the eagle owl and long eared owlwere studied and analysed with comparison. Meanwhile, the noise suppressioncharacteristics of silent flight owl were applied in engineering with bionics theory. The studywas aimed to provide the theory and data basis for understanding the mechanism of owl'ssilent flight and biological model for noise reduction technology.The flight noise of long eared owl in flapping flight and fixed flapping were studied andcompared with sparrow hawk(Accipiter nisus), which has the similar body size and flightmechanism to the long eared owl. Meanwhile, the sound absorption character of the eagleowl's feather were analysed with the impedance tube in1k~6.3kHz. The sound spectrumresults of flight noise showed that the long eared owl flies with lower intensity sound compared to the sparrow hawk, and the flight noise of the long eared owl was mainly at lowfrequency. Compared to the flapping flight, the fixed flapping noise of long eared owl wasbigger at low frequency, but not variety at high frequency. The sound absorption coefficientmeasurement results showed that the feather of the eagle owl has great sound absorptioncharacter, and the absorption of high frequency sound was better than low frequency sound.By covering the feather of the eagle owl on the absorption material, the sound absorptioncoefficient showed that the feather was helpful to improve the absorption character in thefrequency of1k~3.1kHz.The quantitative comparison study between the feather of the eagle owl and commonbuzzard(Buteo Buteo) were carried on macroscopic and microscopic level to explore therelationship between the special characteristic of feather and silent flight. The quantitativeresults showed that the eagle owl has bigger and wider feather, the rachis-barb angle of innervane was bigger than outer vane. From the proximal of feather to the distal of feather, therachis-barb angle of the primary feather of the eagle owl was increased first and thendecrease. The feather of the eagle owl showed obvious asymmetry, which reducing theoverlapping area of adjacent feather and helping to reduce the friction noise. By observationof special characteristics of the eagle owl, the distal barbule of base of serration barb hashooklets, but the distal barbule of serration tip of barb has not hooklets. The fringes on theborder of feather overlapping region and trailing edge of wing have smoothed boundarylayer and reduced boundary layer noise. The barb of the eagle owl had elongated distalbarbule, which was formed a multi-layer grid porous structure on microscopic level anddowny surface on macroscopic level. The multi-layer grid porous structure plays a role ofsound absorption.The study has abstracted the aerofoil of collared scops owl's wing at every10%semi-wingspan, and analysed the airfoil parameters. The results showed that airfoil ofcollared scops owl has thick leading edge and thin trailing edge. From the root of wing to thedistal of wing, the angle of attack of airfoil showed the trend of decreased first and increasethen. The maximum thickness of airfoil was mainly at the range of10%~30%chord. In addition, the maximum chamber of different airfoil has different distribution. From10%semi-wingspan to the90%semi-wingspan, the maximum chamber position has moved fromthe leading edge to the trailing edge. The maximum chamber and thickness of the wing ofcollared scops owl was increased first and then decreased from the root of wing to the distalof wing.The kinematic character of the silent flight owl was studied preliminarily on3D motioncapture technology. The study has compared the flight motion between long eared owl andthe eagle owl, also compared the flight motion between the long eared owl and sparrowhawk. According to the kinematic data, the flight trajectory, flapping frequency, flappingcycle, wrist angle, upstroke and downstroke percentage of cycle were computed andanalysed. The kinematic equation of the flight of long eared owl was establishedpreliminarily. The wing tip, wrist and primary feather P10of long eared owl and the eagleowl have obvious trajectory in flapping flight. Especially the wing tip trajectory in upstrokeshowed double"8"type. However, the wing tip trajectory in upstroke of sparrow hawkshowed single"8"type. The long eared owl and sparrow hawk have similar body size, andhave not obvious difference on flight speed, but the flapping frequency of the sparrow hawkwas bigger than long eared owl. The various trend of wrist angle of the eagle owl and longeared owl have no difference, the wrist angle has kept in downstroke and decreased inupstroke. But the wrist angle of sparrow hawk was increased first and then kept in downstroke, and the angle has begun decrease in upstroke.Based on noise suppression mechanism, the noise suppression characteristic of theeagle owl's feather was applied with the serration and stripe structure, and has reconstructedon the blade of axial fan. The coupling bionic noise reduction fan has designed. According tothe experimental optimization, the influence of serration and stripe structure for aerodynamicnoise of fan blade was studied. The results show that the coupling bionic blade with serrationand stripe structure has lower aerodynamic noise. In the test condition, the A-weightedsound pressure level of bionic fan blade can reduce9.8%at most. The range analysis andvariance analysis results showed that the stripe structure has more important effect on aerodynamic noise reduction of bionic fan than serration structure. The width of stripe, thedepth-diameter ratio and section shape of stripe were main factors for noise reduction ofcoupling bionic fan blade. When the stripe structure had the width of1.5mm, thedepth-diameter of2, the distance of6mm and U shape, and the serration structure had thewidth of3mm, the coupling bionic fan has best noise reduction effect.