When subjected to a steady cross-flow, the deformation of flexible blades is known to result in the alleviation of the internal stresses in comparison to rigid structures. In the field of biomechanics, the flow-induced deformations of flexible structures leading to stress reduction have been often referred to as ‘reconfiguration’ in order to highlight the alleged benefits of such an adaptive process. In this paper, we investigate the reconfiguration of thin elastic blades and the resulting internal stresses when the flow about the blade is oscillatory. Our approach, based on numerical simulations using reduced order fluid force models, is validated by experimental observations. Through a systematic investigation of the response of the structure, we identify four kinematic regimes depending on the excursion of the fluid particles relative to the dimensions of the blade and on the frequency of the flow oscillations relative to the characteristic frequency of the blade. When the flow amplitude is smaller than the structural width, fluid inertia dominates over drag and the fluid–structure coupling triggers resonances that may cause a magnification of the internal stresses. But the small magnitude of the fluid load in this regime is unlikely to cause any severe damage in practice. Otherwise, when drag is the dominant load, flexibility always permits a reduction of the internal stresses. As in the static case, dynamic reconfiguration results in the concentration of the stresses within a small bending length whose scaling depends on the kinematic regime. The magnitude of the stresses does not depend on the actual length of the structure anymore, which suggests the absence of mechanical limitations to the axial growth of wave-swept plants. However, the risk of resonances originating from the inertial load when the blade width compares with the flow excursion favours elongated shapes that best accommodate the oscillatory fluid loadings.

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The static reconfiguration of flexible beams exposed to transverse flows is classically known to reduce the drag these structures have to withstand. But the more a structure bends, the more parallel to the flow it becomes, and flexible beams in axial flows are prone to a flutter instability that is responsible for large inertial forces that drastically increase their drag. It is, therefore, unclear whether flexibility would still alleviate, or on the contrary enhance, the drag when flapping occurs on a reconfiguring structure. In this article, we perform numerical simulations based on reduced-order models to demonstrate that the additional drag induced by the flapping motion is almost never significant enough to offset the drag reduction due to reconfiguration. Isolated and brief snapping events may transiently raise the drag above that of a rigid structure in the particular case of heavy, moderately slender beams. But apart from these short peak events, the drag force remains otherwise always significantly reduced in comparison with a rigid structure.