Poststall hysteresis and flowfield unsteadiness on a NACA 0012 airfoil

The current study was conducted to understand flowfield unsteadiness associated with static stall hysteresis on a NACA 0012 airfoil at Rec = 1.0 × 106. Unsteady pressure measurements were acquired to evaluate the performance of the airfoil, and the presence of a hysteresis loop was identified in the vicinity of the airfoil Cl;max. At poststall angles of attack, two fundamentally different flow regimes were observed between the upstroke and downstroke branches of the hysteresis loop. A Fourier analysis of the surface pressure distributions was used to attribute the flowfield unsteadiness to a low-frequency, high-amplitude oscillation in the flow near the leading edge, along with a regular bluff-body shedding frequency across the separated region of the airfoil. The low-frequency oscillations were observed to be more dominant for the upstroke branch, whereas the bluff-body shedding process was more dominant in the downstroke branch. Time-resolved particle image velocimetry data were acquired across the airfoil to qualitatively and quantitatively describe the low-frequency unsteadiness in the flow. These data were used to link the oscillations across the leading edge during the upstroke to a quasi-periodic surging of the flow, along with an advancement and retreating of the separation location across the surface.