AERODYNAMIC PERFORMANCE AND ACOUSTIC CHARACTERISTICS OF BIONIC AIRFOIL INSPIRED BY THREE-DIMENSIONAL LONG-EARED OWL WING UNDER LOW REYNOLDS NUMBER

摘要

The aerodynamic performance and noise of the blade are two important aspects which people pay much attention nowadays in the design of turbine machinery such as centrifugal fan and axial flow fan. In this paper, the three-dimensional model of the long-eared owl wing is established based on the section theory and fitting formula firstly. And then, unsteady aerodynamic and acoustic characteristics of the bionic blade are numerically investigated using Large-Eddy Simulation (LES) and the Ffowcs Williams-Hawkings (FW-H) equation based on Lighthill's acoustic theory. The results indicate that the deeply concaved lower surface near the wing root plays a significant role in improving the lift-to-drag ratio. The lift coefficient and drag coefficient of the bionic blade is analyzed by comparing two-dimensional and three-dimensional results. The cross section profiles near the wing root possess the larger lift coefficients and the lesser drag coefficients, even than the three-dimensional long-eared owl wing. The size of the separation bubble grows at increasing angle of attack. The 40% cross-section profile of the long-eared owl wing could increase the distance between the corresponding vortex centers with wall surface thus reducing the range of the vortex shedding near the wall effectively. The iso-Q surfaces show that the location of the vortex shedding and the movement of separation bubble. When the angle of attack α is 5°, the aerodynamic noise generated by the bionic blade is lower than in other angle of attack condition. The minimum value of the sound pressure level (SPL) is even 17.9dB on the y-direction. In the range of 5°-15°, the strength and size of the vortex motion increase with the increase of the angle of attack. The far-field noise suggests the directivities of dipole noise. The range of the separation bubbles act as the most influence of the noise generation. The sound pressure level (SPL) of bionic blade at α = 5° is less than other conditions and the minimum value is even 17.9dB. The thin airfoil near the wingtip could decrease the pressure fluctuation from the blade surface that can reduce the unsteady aerodynamic noise. It means the unique structure of the long-eared owl wing can suppresses the unsteady pressure fluctuation on the surface which could decrease the noise generated by the wing surface.