On the transition to 3D motions of wall-mounted flexible plates: insights of the flow and plate interaction with volumetric 4D-PTV

The onset of 3D motions from regular 2D oscillations of wall-mounted flexible plates of low aspect ratios was experimentally inspected for various Reynolds (Re) and Cauchy numbers (Ca) as well as mass ratios and yaw angles under uniform flows. Experiments were performed in an Eiffel-type wind tunnel and a refractive-index-matching (RIM) channel. Combined tracking velocimetry and volumetric particle image velocimetry were used to characterize the unsteady dynamics of the structures and surrounding flow. Complementary force and moment measurements were taken to derive the critical condition triggering the 3D motions of thin plates of rectangular cross sections, which were placed near the inlet of the facilities to minimize the effect of the boundary layer on the motions. Results reveal that the onset to 3D motions occurs at specific dynamical condition, which is satisfied across all cases. The 3D motions of the structures exhibited orbital motions dominated by the structure and torsional natural frequencies. Such motions induced distinctive coherent vortices that, in turn, affected the regular dynamics of the plates leading to unexpected plate velocity patterns.