Piezoelectric modulation of in-line and cross-flow vortex-induced vibrations of a flexible cylinder

Vortex-induced vibration (VIV) of a low-aspect-ratio flexible cylinder ( L / D = 40 ) with embedded spanwise piezoelectric bending actuators is investigated using water-channel experiments and three-dimensional immersed-boundary simulations to determine how a fixed-frequency, open-loop piezoelectric bending perturbation interacts with the underlying unactuated modal state. Measurements over nominal reduced velocities 3 ≤ Vr n ≤ 13 ( Re = 0.9 –5.2 × 10 3 ) resolve coupled in-line (IL) and cross-flow (CF) response for two orthogonal bending orientations. Proper orthogonal decomposition (POD) shows that in the unactuated response, CF motion is governed by the fundamental bending mode throughout, whereas the IL response switches abruptly to the second bending mode near Vr n ≈ 4.3 and returns to the first mode for Vr n ≳ 6. Open-loop actuation produces regime-dependent outcomes: at V r n = 3.75 it nearly eliminates IL motion and reduces CF amplitude by up to 75%, with strongly weakened force fluctuations and loss of near-wake coherence; at V r n = 5.5 the same forcing increases the CF response substantially and restores a clear 2: 1 IL-to-CF frequency relationship. Simulations forced with the measured kinematics show that actuation reshapes the spanwise distribution of velocity-in-phase forcing and drives transitions between aligned and oblique 2 S shedding. Dynamic mode decomposition (DMD) reveals a redistribution of oscillatory wake content among several energetic mode pairs, providing a quantitative link between wake reorganization and changes in IL-CF synchronization. Thus, the structural mode transition is present without actuation, while the same piezoelectric input either suppresses or amplifies the coupled response depending on the baseline state.