Experimental Study of the Periodic Vortex Shedding and Lock-in Region of an NACA4412 Airfoil

Separated shear layer roll-up over the airfoil suction surface at post-stall incidences leads to harmonic vortex shedding and may damage the body structure if the shedding frequency gets locked into one of the natural frequencies of the body. This article presents the results of a series of wind tunnel tests carried out on a stationary and oscillatory NACA4412 airfoil at an angle of attack (AOA) of 45° and Reynolds numbers $Re{=1.34\times 10}^5$ and $Re{=1.52\times 10}^5$ to perform an in-depth investigation of the vortex shedding and lock-in region for an oscillating airfoil in the low subsonic flow regime. Resulting from the semisinusoidal forced pitching motion, the lock-in region in the oscillation’s frequency-amplitude plane was V-shaped, where the vortices shed with the oscillation frequency (lock-in condition) inside the V boundaries. Outside the V boundaries (unlocked conditions), the random shedding of vortices and their interaction causes the kinetic energy scattering in the frequency band limited to the oscillation frequency and the frequency associated with the quasi-steady condition due to the mean angle of attack. Moreover, the aerodynamic damping was examined by calculating the net energy transfer between the body and flow indicated that the direction of the energy transfer between the airfoil’s body and flow is a function of both the oscillation amplitude and frequency, except during oscillation with the resonant frequency at which energy is always transferred from flow to the body.