Multi-Scale Vortex-Induced Vibrations of a Circular Cylinder in Combined Steady and Oscillatory Flows

This study examines vortex-induced vibrations (VIV) of a circular cylinder in flows composed of a steady component with superimposed sinusoidal oscillations. Water-channel experiments were conducted to quantify the cylinder response and wake dynamics across a range of reduced velocities, oscillation amplitudes, and frequency ratios. Results reveal that incoming flow oscillations induce distinctive multi-scale vibration patterns characterized by a dominant frequency and two sidebands, corresponding to a beat mechanism between vibration frequency and the imposed flow oscillation frequency. These non-quasi-static interactions lead to substantial deviations from quasi-static VIV predictions, particularly in the upper branch region, where amplitude enhancements up to 50% were observed. Phase analysis revealed hysteresis and intermittent switching between classical VIV branches within a single flow oscillation cycle. Particle Image Velocimetry (PIV) measurements showed corresponding wake transitions among 2S, 2P, and mixed shedding modes. A reduced-order model was developed to capture the observed multi-frequency response and its dependence on oscillation intensity and frequency ratio.