Flow-induced rotational vibrations of an inverted rigid cylinder-plate: Vortex-induced vibration in disguise?

We numerically investigate the rotational flow-induced vibration (FIV) of an inverted rigid cylinder-plate in a 2D space at a Reynolds number of 100. The present fluid-structure interaction (FSI) system consists of a torsional spring that is attached to the cylinder center while the plate is oriented to the windward side. A forced torsional harmonic oscillator equation is coupled to the in-house sharp-interface immersed boundary method-based flow solver for simulations. Several FIV regimes, namely initial excitation, intermediate and final symmetry-breaking, large amplitude flapping (LAF) and chaos, are observed with increasing reduced velocity. We propose a quasi-static aeroelasticity assumption-based model to explain these FIV regimes and quantify modifications in the structural natural frequency. The underlying mechanism of the LAF is found to be vortex-induced vibration (VIV) since the cylinder-plate oscillation frequency synchronizes with its natural frequency, suggesting lock-in. The other probable mechanisms for LAF, viz. flutter and torsional galloping, were also tested and ruled out. The current system's energy harvesting potential is found to be better than the translational VIV of a circular cylinder under the same flow conditions.