Flow over a cylindrical structure is a problem of interest in different engineering fields, especially offshore structures like risers and spars, which are constantly subject to sea waves. In this study, numerical simulation of the flow over a fixed cylinder at a low Reynolds number was carried out using the Reynolds-averaged Navier-Stokes model. It is well known theoretically and experimentally that the lift is self excited and has a frequency equal to the vortex shedding frequency. Then the cylinder was forced to oscillate in the cross-stream direction with different frequencies and amplitudes. The flow structure, including the lift and drag, was examined within the so-called lock-in frequency range, where the lift frequency is entrained by the cylinder frequency. The frequency-response curves for the lift and drag and the phase between the hydrodynamic force and the cylinder were produced for different motion amplitudes. These curves change significantly as either the motion amplitude or frequency is varied. Two modes of response were identified associated with high lift and low lift; each one has different characteristics.

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