| The detailed behaviour of the unsteady boundary layer and stall events occurred on a sinusoidally oscillating NACA 0012 airfoil at Re = 1.35 x 105 was investigated experimentally by using closely-spaced multiple hot-film sensor arrays. The hot-film measurements were supplemented by surface pressure measurements, hot-wire wake velocity surveys and smoke-flow visualizations. Three typical oscillation parameters: attached flow, light stall, and deep stall were tested. Special attention was focused on the non-intrusive identification of the spatial-temporal progression of the locations of the boundary-layer transition and separation and reattachment points for a range of oscillation frequencies and amplitudes both prior to, during, and post the stall. The results show that for an unsteady airfoil, the reduced frequency of the oscillation was found to be highly significant and only small values of reduced frequency were required to delay the onset of the various boundary-layer events, and to produce significant variations and hysteresis in the peak values of lift and drag forces and the pitching moments, which are fundamentally different from their static counterparts. Lift stall was observed to occur when the leading-edge vortex reached 90% of the chord, while moment stall occurred at the end of the upward spread of the trailing-edge flow reversal. The convection speed of the leading-edge and the secondary vortices were also reported. Dynamic stall was found to be caused by an abrupt turbulent separation near the leading-edge region and not with the bursting of the laminar separation bubble, as is commonly observed for a static NACA 0012 airfoil; the result being the initiation, growth and convection of an energetic leading-edge vortex. Moreover, the detection of the sudden turbulent breakdown could serve as an indicator for dynamic stall detection and control. The results reported on here provide a deepened insight into the detailed nature of the unsteady boundary-layer events as well as the stalling mechanisms at work at different stages in the dynamic-stall process. |
