Experimental Small-Amplitude Self-Sustained Pitch–Heave Oscillations at Transitional Reynolds Numbers

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Experimental Small-Amplitude Self-Sustained Pitch–Heave Oscillations at Transitional Reynolds Numbers

Poirel, D. / Mendes, F.

An experimental investigation of a free-mounted rigid NACA0012 airfoil in pitch–heave is conducted at transitional Reynolds numbers. Previously, small-amplitude self-sustained oscillations in pure pitch were observed for a specific range of Reynolds numbers: $5.5 \times 10^{4} - 1.25 \times 10^{5}$ . They were attributed to laminar boundary-layer separation, which leads to negative aerodynamic damping at a 0 deg angle of attack. In this phase of the research, the investigation is extended to 2 deg of freedom by enabling the heave motion. Depending on initial conditions, two competing stable limit-cycle oscillations, small and large, are observed. It is determined that the origin of the large-amplitude limit-cycle oscillations is coalescence flutter for which the exponentially growing amplitude is limited by flow separation at large angles of attack. On the other hand, the small-amplitude oscillations are the same laminar separation flutter type as the pitch-only case. For the most part, the heave motion plays no fundamental role in the dynamics, as the amplitude of the pitch oscillations is not drastically affected in comparison with the single-degree-of-freedom pitch-only case nor is the range of Reynolds numbers at which the limit-cycle oscillations are observed. Nonetheless, the heave motion contributes to the net transfer of energy from the flow to the structure, as the mean kinetic energy present in the oscillations is increased significantly compared to the pitch-only case.