Vortex-induced vibration mechanism of the NACA 0012 airfoil based on a method of separating disturbances

Vortex-induced vibration (VIV) is the result of the interaction between the airfoil vibration and vortex shedding, and it frequently achieves the peak vibration amplitude in the lock-in region. Therefore, distinguishing the effect of the two disturbances in the lock-in region is important for understanding VIV. In this study, the NACA 0012 airfoil was investigated at high angles of attack; thus, the vortex could form and shed regularly downstream. The VIV was analyzed numerically and verified using the test data in references. The process of the vortex shedding was observed to be similar under different incoming flows, vibration amplitudes, and frequency ratios. In the V-shape lock-in region, the phase between the airfoil vibration and vortex shedding changes with the frequency ratio. When the frequency ratio is 1, the phase and strength of the vortex shedding remain almost constant with the vibration amplitude. Based on the features of the vortex shedding, a method of separating the disturbances caused by the airfoil vibration and the vortex shedding is proposed to investigate the mechanism of VIV from the perspective of energy balance. As the phase between the airfoil vibration and vortex shedding changes in the lock-in region, the vortex shedding provides negative aerodynamic damping (AD) in some phases and positive AD in other phases. The stability of the airfoil depends on the two disturbances mentioned above. VIV occurs only when the disturbance of the vortex shedding performs positive work (negative AD) and the disturbance of the airfoil vibration performs negative work (positive AD). When the vibration amplitude is small, VIV is dominated by the disturbance of the vortex shedding. When the vibration amplitude is high, the VIV is dominated by the disturbance of the airfoil vibration. As the vibration amplitude increases, the two form a balance, and the airfoil exhibits limit cycle oscillations. Based on the assumption of linear superposition, the theoretical limit vibration amplitudes of the airfoil under the two conditions in this study were 3° and 13.4°, respectively.