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

Vortex shedding resonance or lock-on is observed when a circular cylin der is excited into rotational oscillations. Boundary element numerica l simulations of this flow at subcritical Reynolds numbers of 1500 and 3000 are computed. Two lock-on or resonance flow regimes are predicte d. The primary lock-on regime occurred when the reduced forcing freque ncy was in the vicinity of one, while the tertiary lock-on was observe d at a reduced forcing frequency around three. The frequency bandwidth s of the two lock-on regimes are found to be correlated with the reduc ed rotational amplitude of the oscillating cylinder. Behavior of the a erodynamics forces during the lock-on regimes is investigated. It was found that it is the sharp variation of the phase shift between the ph ase of the velocity oscillation of the body and the phase of the initi al vortex formation, that causes the remarkable increase in the aerody namic forces within the lock-on regimes. The global subcritical lock-o n characteristic diagram for a rotationally oscillating cylinder is ob tained. The present BEM numerical model is verified by a series of tes ts. The tests include mesh-size refinement studies, comparisons with e xperiments and other solutions for a variety of benchmark problems and for a wide range of physical parameters.