Mitigating unsteady loads at low Reynolds numbers using a passive trailing-edge flap

The load mitigation potential of a passive pitching trailing edge flap for NACA0012 airfoils at a Reynolds number of 1000 subjected to oscillations in the angle of attack is analyzed. For this purpose, direct numerical simulations of the two-dimensional, incompressible flow have been conducted to examine the effectiveness of the flap in reducing aerodynamic load fluctuations across a range of oscillation amplitudes and flap-to-chord ratios. The validity of a quasi-steady model to predict the load mitigation using passive pitching flaps, previously proposed in the literature and predicting a load mitigation proportional to the flap-to-chord length ratio, a/c, is here investigated for large amplitude oscillations. The results show that the increment in the reduction in fluctuations is generally proportional to the increment in a/c. This closely aligns with the predictions of the quasi-steady theory, even for the cases with the largest oscillation amplitudes, where non-linear aerodynamic effects are present, although some variation is observed. Notably, we explored the interaction between vortical structures and the flap dynamics, and its relevance on the flow patterns around the airfoil and ultimately on load mitigation. This interaction, alongside flap inertia, provides insight into the timing and magnitude of load reduction, demonstrating the potential of tailored passive pitching mechanisms for unsteady flow conditions. These findings offer valuable insights for the design and development of passive unsteady load mechanisms for small aerial and underwater vehicles, as well as microscale energy harvesters, by highlighting the relevance of considering non-linear effects in their optimization.