The aeroelastic dynamics of a NACA 0012 airfoil oscillating in pitch at transitional Reynolds numbers

The aeroelastic dynamics of a NACA 0012 airfoil oscillating in the pitch degree of freedom were investigated experimentally at chord-based Reynolds numbers from 4x104 to 1.4x105. The experiments were carried out using an instrumented test rig housed in the test section of a low turbulence, subsonic wind tunnel. The rig included a spring-supported NACA 0012 airfoil with a chord of 0.156 m that was mounted vertically in the test section.;In the two forwardmost elastic axis positions, limit cycle oscillations were observed at airspeeds between 4 m/s and 13 m/s. The oscillations, which could not be sustained at airspeeds outside this range, had amplitudes of less than 6° and frequencies between 1 Hz and 5 Hz. With stepwise increases in airspeed, the amplitude of the pitch motion increased up to a maximum, after which it decreased gradually until the motion ceased. The frequency, meanwhile, generally increased with airspeed, with the largest increases occurring at the highest airspeeds. The amplitude and frequency of the oscillations were also found to depend on structural stiffness. The 0 N·m setup typically had the largest amplitudes and lowest frequencies, whereas the smallest amplitudes and highest frequencies tended to occur in the 0.30 N·m setup.;In the rearmost elastic axis position, multiple attractors were noted in the response of the airfoil. With a structural stiffness of 0.15 N·m in this configuration, limit cycle oscillations with amplitudes of less than 4° were observed at airspeeds up to 8.6 m/s. This was followed by a change to oscillations with amplitudes between 21° and 23° from 10.4 m/s onwards. Between 8.6 m/s and 10.4 m/s, the attractors coexisted and either type of motion was possible. In the 0 N·m setup, the airfoil exhibited a non-periodic response at airspeeds below approximately 7 m/s, after which large amplitude oscillations emerged, similar to those witnessed at the higher airspeeds in the 0.15 N·m setup.;For the routinely observed oscillations, namely those whose amplitudes did not exceed 6°, an analysis of the unsteady, viscous aerodynamic moment coefficient about the elastic axis revealed the effective aerodynamic stiffness to have a combined Reynolds number and pitch angle dependence. The analysis also predicted the effective aerodynamic stiffness of the two forwardmost elastic axis positions to be positive at all airspeeds in the oscillatory range, whereas in the rearmost elastic axis position, a negative effective aerodynamic stiffness was predicted for airspeeds below about 8 m/s in both the 0.15 N·m and 0.30 N·m setups.;This setup enabled the aeroelastic response of the airfoil to be studied parametrically. In particular, the airspeed, elastic axis position, and structural stiffness were investigated. Of the three elastic axis positions that were considered, two were located ahead of the quarter-chord point and the third was located slightly aft of the quarter-chord point. Three values of structural stiffness were also considered, namely 0 N·m, 0.15 N·m, and 0.30 N·m.;The results of hot-wire tests in which the airfoil was held static suggest that the oscillations are self-excited since no pitch motion periodicity was detected either in the flow upstream or in the wake downstream of the airfoil. Physically, the oscillations are suspected to be related to the strong viscous effects inherent to the transitional Reynolds number regime. In particular, they are speculated to result from laminar trailing edge separation and the ensuing formation of a laminar separation bubble on the suction surface of the airfoil, which together lead to a negative aerodynamic damping type instability.