EXPERIMENTAL STUDY OF DYNAMIC AERODYNAMIC SWEPT EFFECT ON YAW OSCILLATING AIRFOIL

The design of large wind turbines has put forward higher requirements for obtaining more comprehensive and accurate dynamic loads of airfoil. It is of great significance to study the influence of yaw oscillation on the dynamic aerodynamic characteristics of airfoil. With the help of "electronic cam" technology and synchronous acquisition of dynamic data, the wind tunnel test of yaw oscillation was first carried out for the dynamic "sweep effect" of the airfoil in this paper. The study shows that the aerodynamic curve of the yaw oscillating airfoil has obvious hysteresis effect, the periodic pressure fluctuation of the airfoil suction surface is the main inducement, and the aerodynamic hysteresis characteristics are enhanced with the increase of the oscillation frequency, the initial angle of attack and the amplitude. The hysteresis loop of the lift and pressure difference drag changing with yaw angle is "W" type, the hysteresis loop of the pitching moment is "M" type, and the hysteresis loop of the lift difference is "\infty" type. The aerodynamic force of the airfoil under the negative stroke is higher than that under the positive stroke, and the aerodynamic coefficients of the airfoil under negative stroke increase slightly with the increase of the oscillation frequency, but decrease obviously under positive stroke with the increase of the oscillation frequency. The power spectral density (PSD) distribution of the airfoil lift coefficient has obvious energy concentration characteristics at the integer multiple of the oscillating frequency, and with the increase of the oscillation frequency, the amplitude of the energy concentration is obviously increased, which reflects the enhancement of the unsteady flow around the yaw motion airfoil. There is hysteresis effect of the pressure coefficient changing with the yaw angle on different suction surface position, in which the hysteresis areas of the pressure coefficient are larger on 1.2% chord position and 40% chord position, as the result of the periodic generation, development, movement, breakdown and reconstruction of the shear layer vortex and the dynamic separation vortex on the airfoil surface. When the amplitude of yaw oscillation \beta_1=10^\circ, the shape of C_\text L-\beta curve is "^\wedge" type. When the amplitude of yaw oscillation \beta_1=30^\circ, the shape of C_\text L-\beta curve is "^\wedge\wedge\wedge" type, that is, when the yaw angle \beta>20^\circ or \beta<-20^\circ, the shape of the lift coefficient hysteresis loop of the airfoil presents "drooping" phenomenon.