Reduction of the Loads on a Cylinder Undergoing Harmonic In-Line Motion

We use the finite-difference computational fluid dynamics method to study in detail the flow field around a circular cylinder in a uniform stream while undergoing in-line harmonic motion. For a given motion amplitude, there exists a critical forcing frequency below which the lift and drag can be period-n, quasiperiodic, or chaotic. Similarly, for a given frequency, there exists a critical amplitude below which the lift and drag can be period-n, quasiperiodic, or chaotic. Above these critical conditions, the lift and drag are synchronous with the forcing. The lift nearly vanishes and the mean drag drops and saturates at a value that is independent of the driving frequency, whereas the oscillatory drag quadratically depends on it. We relate these features to changes in the wake and the surface-pressure distribution. We examine the influence of the Reynolds number on these critical frequency and amplitude. Second- and higher-order spectral analyses show remarkable changes in the linear and quadratic coupling between the lift and drag when synchronization takes place; it destroys the two-to-one coupling between them in the cases of no motion and synchronization due to cross-flow motion. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3210774]