Experimental Evaluation of the Cycle-to-Cycle Variations Observed in Dynamic Stall

Dynamic stall, an unsteady aerodynamic phenomenon characterized by rapid changes in incoming flow direction, is a subject with significant implications for rotorcraft flight and wind energy generation. Traditional experimental research in this field involves pitching or plunging an airfoil through hundreds of cycles and phase averaging the resulting unsteady pressure data. This approach is appropriate when data at all times during a cycle can be roughly approximated by the cycle ensemble average. Recent studies, however, have demonstrated that downstroke flow behavior may sometimes be statistically multimodal, which would inhibit the use of phase averages for effective analysis. The objective of this study is to understand the nature of cycle-to-cycle variations through a parametric survey involving surface pressure measurements. An array of experimental data was collected for various airfoil pitching schedules on an SC1094R8 model. Parameters that varied in the study included the mean and amplitude angles of oscillation and the oscillation frequency. The effect of each of these parameters on stall sensitivity is considered, along with the influence of cycle-to-cycle variations on aerodynamic lift and moment. The results identify bifurcation in separation and reattachment patterns under certain, predictable conditions, where it is observed that phase averages misrepresent the actual dynamic stall behavior.