Meandering-driven flapping of a flexible flag in a turbulent free-surface water jet

In this work, we study the interaction and coupled dynamics between a turbulent water jet and a light, flexible Mylar flag. Since the classical self-sustained flag flutter is not easily induced for low-inertia flags immersed in water jets, due to the additional stabilizing contributions of viscous effects, we instead explored the possibility of triggering flag oscillation and flapping through an externally imposed flow unsteadiness. Unlike previous approaches based on vortex shedding in the wake of bluff bodies, here the forcing mechanism is provided by the jet’s intrinsic meandering instability. In fact, once sufficiently developed, the meandering motion exhibits characteristic frequencies that are relatively stable along the streamwise direction and can be tuned through the inlet flow rate, making it a suitable candidate for imposing organized oscillations on flexible structures. Our experiments show that, in this way, flapping is accessible, also in water, at very low velocities, and that the flag’s oscillations are strongly synchronized with the meandering dynamics of the jet. The dominant frequencies extracted from the flag motion match those measured in the free jet at the corresponding flow rates, while the amplitude of the flag oscillation increases along the flag length, indicating a progressive development of the deformation from the leading edge toward the trailing edge. This interpretation is further supported by the POD analysis, which shows that the flag response is dominated by a small number of coherent modes, whereas the free jet velocity field retains a broader modal content. Despite this difference, the dominant temporal and convective scales of the flag and of the jet remain close, confirming that the flag behaves as a selective structural filter of the turbulent forcing imposed by the meandering jet.