The Airfoil Flow Characteristics And The Aerodynamic Noise Mechanism At Low To Medium Reynolds Numbers

The aerodynamic performance and aeroacoustic characteristics of the airfoil are the fundamental parameters for the design of various types of fluid machinery such as airplanes,engines,and wind turbines.Small scale wind turbines and micro-mini drones are popular in recent years,which are usually operated at low to medium Reynolds numbers and complex flow conditions,in which the arifoil encounters local stall.In addition,the noise performance has attracted more and more attention.Therefore,study of airfoil flow characteristics and aeroacoustic mechanism from attached to stall flow conditions at low-to-medium Reynolds numbers is of great significance for the optimal design of these related aerodynamic components.In this paper,the NACA 0012 airfoil is selected as the experimental model,and its aerodynamic characteristics,the far-field noise features,the flow field obtaiend by Particle Image Velocimetry(PIV),and the correlation between flow field and noise at different angles of attack are studied.The hysteresis effects of the airfoil was studied by increasing and decreasing the angle of attack.The experiments were conducted in the open-jet test section of the aeroacoustic wind tunnel at Mechanics and Aerospacer Engineering Department,Southern University of Science and Technology.The experiment surveyed the in the trend of the aerodynamic performance and aeroacoustic characteristics of the airfoil during versus the angle of attack at the inflow speed of 10 m/s and 25 m/s,and the effects of the inflow velocity on the noise characteristics of airfoil when the effective angle of attack is 0°,2.56°,4.90°,10.55°,14.28°,16.22° and 23.21°.The far-field noise and flow field measurements were syschrolized and analized for the noise generation mechanism.NACA 0012 airfoil features the typical leading-edge stall characteristics.The flow structures show obvious symmetrical events when the effective angle of attack is 0°.The trend of lift coefficient versus angle of attack is consistent with that of the velocity measured by PIV around the airfoil.Through measurements of the instantaneous flow field,the convection and trajectory of the vortex structure on the airfoil surface can be clearly observed.The far-field noise measured at 10 m/s is mainly concentrated in the low-frequency part of 100 ? 1000 Hz,and the sound pressure level in the middle and high-frequency part is always maintained at about 0 d B.There is obvious tonal noise when the effective angle of attack is 0° small angles of attack.After an effective angle of attack reaches 10.55 °,the overall sound pressure level shows a clear linear increasing trend versus the flow velocity.When the inflow velocity is 25m/s,there is no tonal noise observed at 0)0)0)0)0)0)= 0°,and the consistency between the overall sound pressure level and the lift coefficient at large angles of attack is more obvious.The frequency spectrum of the airfoil flow field characteristics and the far field noise characteristics under different conditions at an incoming flow velocity of 10 m/s were compared and analyzed.By comparing the velocity fluctuation spectrum to the noise spectrum,it was found that the low frequency tonal noise generated at a small angle of attack is mainly caused by the velocity fluctuation of the airflow near the trailing edge.In the symmetric flow conditon,the periodic flow on the pressure and suction sides of the airfoil both contribute to tonal noise.In the asymmetric flow condition,the airfoil tonal noise is mainly contributed by the periodic flow on the pressure side.During the airfoil stall condition,there is no tonal noise on the airfoil.consistently,only the broad-band characteristics of turbulent flow near the leading edge and trailing edge of the airfoil were observed,and no periodic flow characteristics were detected.