Precursors to flutter instability by an intermittency route: A model free approach

•Development of precursors to experimental flutter presented.•Intermittent bursts observed ahead of experimental flutter.•Repeating patterns in intermittent response visualized in recurrence plots.•Statistics of recurrence states used to develop precursors to forewarn flutter.•Numerical study with a canonical wind model used for validating precursors.

The aeroelastic response of a NACA 0012 airfoil in the flow regimes prior to flutter is investigated in a wind tunnel. We observe intermittent bursts of periodic oscillations in the pitch and plunge response, that appear in an irregular manner from a background of relatively lower amplitude aperiodic fluctuations. As the flow speed is increased, the intermittent bursts last longer in time until eventually transitioning to a fully developed periodic response, indicating the onset of flutter. The repeating patterns in the measured response are visualized using recurrence plots. We show that statistics of the recurrence states extracted from these plots can be used to develop model-free precursors that forewarn an impending transition to flutter, well before its onset.