In this study, the velocity field in the wake behind a rigid airfoil mounted on a flexible support is analyzed. The main focus was on the wake structures generated in the flow behind the airfoil during the pitching and heaving motions of the airfoil resulting from the feedback interaction of the flow and the structure. The airfoil used is a NACA0012 and has a cord of 0.156 m and a span of 0.6 m between the end plate resulting in an aspect ratio equal to 3.9. The elastic axis was located at 35% of the chord length from the leading edge. Hot wire anemometry was used to record velocity fluctuations along the main flow direction. Velocity fluctuations were analyzed to determine the effect of the airfoil oscillations on the structure of the wake flow. The study was carried out at different Reynolds numbers in the range of 7.5 x 104 ≤ Rec ≤ 1.3 x 105 where large amplitude self-sustained oscillating motions in pitch and heave are observed.

Few reported experiments have focused on nonlinear aerodynamic-structure interaction phenomena occurring at low to moderate Reynolds numbers on an elastic wing. In fact in the range of Reynolds numbers, 104 ≤ Rec ≤ 106, complex viscous phenomena occur such as laminar boundary layer separation leading to the formation of a laminar separation bubble, transition of the laminar shear layer, and subsequent reattachment of the turbulent layer. It is clear that in this range of Reynolds numbers, the flow structure has a significant impact on an elastic airfoil behavior and on the aero elastic phenomenon. In this paper, wake vortex shedding is analyzed during the airfoil oscillation in pitch only and in pitch and heave together.

The apparatus is a two-degree-of-freedom system, composed of a rigid wing moving in translation (heave) and in rotation (pitch). The apparatus is capable of exhibiting fundamental wing aero elastic phenomena, both linear and nonlinear, such as flutter, limit cycle oscillations (LCO) and response to external turbulence. In order to measure both the airfoil motion and the velocity simultaneously, 2 potentiometers and a hotwire probe were used. This probe was mounted to measure the longitudinal (u) component of the velocity in the wake of the airfoil at one cord length behind the airfoil. Velocity was measured using a DISA CTA bridge 56C17. The velocity fluctuations and pitch (or pitch and heave) signals were conditioned using a WBK18 and an 8 channel dynamic signal conditioning module and digitized with a 16 bit AD Wavebook from IOTEC with a sampling frequency of 5 kHz. 

Self-sustained oscillations were observed for a range of Reynolds numbers mentioned above. The amplitude of oscillations of the airfoil was determined as a function of air speed. It ranged from 32° to 47° for the pitch and from 1.5 cm to 3 cm for the heave. The frequency of both the pitch and heave increased almost linearly with air speed from 2.5Hz to 3.2Hz.

The wake structure is documented via power spectra of u velocity. The case where the airfoil is maintained static is also documented. For this latter case, the signature of the von Kármán vortex shedding street is observed in the entire range of Reynolds numbers studied. The signature of the airfoil motion on velocity field is very clearly noticeable. The low frequency vortex resulting from the airfoil motion has a clear consequence of the appearance of on the normal vortex shedding phenomenon.