EXPERIMENTAL CHARACTERIZATION OF THE TRANSITION BUBBLE ON A NACA0012 OSCILLATING AIRFOIL

The main goal of the present work is to study qualitatively and quantitatively the physics of unsteady flows around a NACA0012 pitching airfoil in 2D and 3D configurations. A particular attention is paid to the boundary layer and the dynamic structure of the flow field when a bubble is formed, grows and bursts as the angle of attack is increasing and decreasing. The experiments were carried out on two rectangular wings of NACA0012 airfoil section, one spanning the entire test section of the wind tunnel (2D configuration), the other spanning only the half section (3D configuration). During the oscillation, the local incidence of the airfoil is varying around mean values α0=6deg and 12 deg through an incidence amplitude Δα=± 6deg, at a reduced frequency k=0.188 and at nominal Reynolds numbers Re=105 and 2.105. The present paper gives some examples of typical results concerning the transition bubble occurring on the upper side of the pitching airfoils. Phenomena occurring in the region very close to the airfoil, like transition, separation, and dynamic stall are also analyzed by means of visualization of the flow and instantaneous velocity measurements.