Aerodynamic, Acoustic Characteristics Of Long-eared Owl Wing And Its Bionic Application

At present, energy crisis and environmental pollution have become two major urgentproblems that need to be solved. Energy saving and noise reduction become more importantwith High-power and high-speed machine like high speed train, private plane increasingquickly. Improving aerodynamic performance and reducing aerodynamic noise are becomingthe front of air power mechanical research.Many birds with elongated wings, such as the albatross(Diomedea exulans), or birdswith long and broad wings, such as strigiformes and falconiformes fly efficiently for goodaerodynamic characteristics of wings. Moreover, many birds of strigiformes, like long-earedowl(Asio otus), tawny owl(Strix aluco) and eagle owl(Bubo Bubo) have the ability to flysilently. During hunting, owls reduce flight speed; wing beat frequency and amplitude inorder to detect their prey and probably to reduce noise emission. That is hard for most of theother birds. Such birds require more flight power for wing movements to produce enough liftwhile flying slowly. Owls with special wing sections and structure of feather in gliding andflapping flight generate lower intensity sound. The silent flight of owl has given a new ideaand method for aerodynamic performance improvement and noise reduction of fan blade andaircraft wing.Long-eared owl and Sparrowhawk are also perfectly adapted to gliding flight;Long-eared owl can fly silently, while Sparrowhawk can’t. Based on the bionics theory, thecomparative analysis of the geometry morphological features of the Long-eared owl and theSparrow hawk feathers were studied. Static three-dimensional wing models and physicalmodels were established, with the help of wind tunnel test and numerical simulation,aerodynamic and acoustic characteristics of the wings were studied. The mechanism ofexcellent aerodynamic characteristics of Sparrowhawk and owl’s silent flight wereunderstood preliminary. The bionic optimized axial fans were designed, the test showed that the efficiency of fan was improved and the noise reduced.The geometric parameters of Long-eared owl and Sparrowhawk were measured. Themeasurement results showed that the leading edge of owl was thicker and the trailing edgewas thinner, the thickness was mainly concentrated in the root of wing. The airfoil thicknessand camber decreased rapidly from the root to the end portion of the owl wing, while theSparrow hawk wing reduced gently. The outermost primaries on leading edge have the combtooth structure and the trailing edge have fringe-like edges, while Sparrow hawk wingfeather didn’t have these features. There were radial V-grooves on the surface of two wings.In this paper, the airfoil geometric parameters of owl and hawk wing were extracted,and the smooth wing models were reconstructed. Aerodynamic performance of wing werecalculated and analysis by Profili software. The aerodynamic performance of hawk wing wasbetter than the owl’s. The airfoil lift coefficient of owl wing decreased larger from root to theend of wing, but the changed trend of lift-drag ratio were the same, the lift-drag ratio of40%cross-sectional airfoil was largest. We found that the aerodynamic characteristic of the airfoilwas affected by the thickness distribution, camber and other factors.An opening low turbulence wing tunnel for testing wing aerodynamic characteristicswas built. The lift and drag test were made at two speeds,2m/s and20m/s. The maximumangle of attack of owl wing at2m/s and20m/s was28°and30°respectively, while hawks’were the same of28°. With the increase of speed, the stalling angle of attack increased.There was less bounder layer separation at high speed for Long-eared owl wing. With thespeed increasing, the lift coefficients of Long-eared owl and hawk were also improved, andthe growth rate in small angle of attack was significantly greater than the high angle of attack.Drag coefficient decreased with speed increasing, but the growth rate at20m/s was smallerthan the growth rate at2m/s. The rotating test was made at the line speed of20m/s, andaerodynamic noise of wing model was measured. Long-eared owl and Sparrowhawk wingmodels mainly produced low-frequency noise. Aerodynamic noise of Long-eared owl wingswas significantly lower than the Sparrow hawk wing model’s.In order to determine the effect of the comb tooth shape parameters on aerodynamic andacoustic performance of Long-eared wing model, the regressive analysis of orthogonal multinomial were performed under20m/s speed and20°angle of attack. The results showedthat the leading edge comb teeths reduced the aerodynamic noise of Long-eared owl wingeffectively. The maximum value of noise (A) reduced8dB with leading edge comb teeths.The results of the range analysis showed that the main factor which influence theaerodynamic and acoustic characteristics was angle with spanwise(β), followed by toothwidth(d) and tooth height(h) had the smallest impact. The best combination of the factorsresulting in the lowest noise was: β=30°, d=1mm, h=1mm. Through the regression analysis,the regression equation between noise reduction and the teeth dimensions (β, d, h) wasobtained.Numerical analysis was made in two speeds,2m/s and20m/s. The results showed thatthe boundary layer on the trailing edge surface of the root wings more likely to occurseparation under small angle of attack due to big curvature and thickness. Wing tipseparation took place close to the leading edge under high angle of attack. The boundarylayer separation increased the boundary layer vortex shedding noise. The boundary layerseparation area of Sparrowhawk wings was greater than the Long-eared owl wing’s, so theaerodynamic noise of the Sparrowhawk was larger. The comb teeths on the leading edgeacted as vortex generators defered the boundary layer separation of wing surface. The airfoiland the comb structures were the main factors for owl silent flight obviously. The pressuredifference of upper and lower surface of hawk was larger than owl’s, thus lift coefficient ofhawk was larger than owl’s in the same conditions.V-shaped and circular section were designed on axial fan flat blade, based on theLong-eared owl and Sparrow hawk wings, the test results showed that, V-shapedcross-section was better than arc-shaped cross-section, the aerodynamic performances of thebionic fans were better than the prototype fan, the maximum flow increased by6.1%, themaximum static pressure increased by7.0%. The outlet flow of bionic fans was more stableand closer to the axis, total pressure was significantly greater. The inverse involute groovesreduced the loss of flow and made the outlet flow closer to axis.Based on characteristics of Long-eared owl and Sparrow hawk wing, six bionic fanswere designed. Tests showed that aerodynamic performance of fan with40%cross-sectional airfoil of hawk was the best, but the aerodynamic noise was larger relatively. Theaerodynamic characteristics of fan with Airfoils changed from the blade root to tip likeLong-eared owl wing was only slightly improved, but aerodynamic noise reducedsignificantly. The40%cross-sectional airfoil camber was modified like the development ofthe Long-eared owl wing, and test results showed that this bionic fan had a goodaerodynamic performance and acoustic performance at the same time.