Self-sustained aeroelastic oscillations of a NACA0012 airfoil at low-to-moderate Reynolds numbers

A wind tunnel experimental investigation of self-sustained oscillations of an aeroelastic NACA0012 airfoil occurring in the transitional Re regime is presented. To the authors' knowledge this is the first time that aeroelastic limit cycle oscillations (LCOs) associated with low Re effects have been systematically studied and reported in the public literature. While the aeroelastic apparatus is capable of two-degree-of-freedom pitch-plunge motion, the present work concerns only the motion of the airfoil when it is constrained to rotate in pure pitch. The structural stiffness is varied as well as the position of the elastic axis; other parameters such as surface roughness, turbulence intensity and initial conditions are also briefly discussed. In conjunction with the pitch measurements, the flow is also recorded using hot-wire anemometry located in the wake at a distance of one chord aft of the trailing edge. It is observed that for a limited range of chord-based Reynolds numbers, 4.5 x 10(4) <= Re-c <= 1.3 x 10(5), steady state self-sustained oscillations are observed. Below and above that range, these oscillations do not appear. They are characterized by a well-behaved harmonic motion, whose frequency can be related to the aeroelastic natural frequency, low amplitude (theta(max)< 5.5 degrees) and some sensitivity to flow perturbations and initial conditions. Furthermore, hot-wire measurements for the wing held fixed show that no periodicity in the undisturbed free-stream nor in the wake account for the oscillations. Overall, these observations suggest that laminar separation plays a role in the oscillations, either in the form of trailing edge separation or due to the presence of a laminar separation bubble. (c) 2008 Elsevier Ltd. All rights reserved.