VORTEX LOCK-ON FOR A ROTATIONALLY OSCILLATING CIRCULAR-CYLINDER - A BEM NUMERICAL STUDY

Vortex shedding resonance or lock-on is observed when a circular cylinder is excited into rotational oscillations. Boundary element numerical simulations of this flow at subcritical Reynolds numbers of 1500 and 3000 are computed. Two lock-on or resonance flow regimes are predicted. The primary lock-on regime occurred when the reduced forcing frequency was in the vicinity of one, while the tertiary lock-on was observed at a reduced forcing frequency around three. The frequency bandwidths of the two lock-on regimes are found to be correlated with the reduced rotational amplitude of the oscillating cylinder. Behavior of the aerodynamics forces during the lock-on regimes is investigated. It was found that it is the sharp variation of the phase shift between the phase of the velocity oscillation of the body and the phase of the initial vortex formation, that causes the remarkable increase in the aerodynamic forces within the lock-on regimes. The global subcritical lock-on characteristic diagram for a rotationally oscillating cylinder is obtained. The present BEM numerical model is verified by a series of tests. The tests include mesh-size refinement studies, comparisons with experiments and other solutions for a variety of benchmark problems and for a wide range of physical parameters.