A numerical study was made of the flow around a circular cylinder in a uniform flow. The cylinder was rotated sinusoidally in time at a forcing frequency ($S_f$) and amplitude ($θ_max$). The temporal behavior of vortex shedding patterns was scrutinized over broad ranges of the two externally specified parameters, i.e., the dimensionless forcing frequency, (0.110 ≤ $S_f$ ≤ 0.220), and the maximum angular amplitude of oscillation ($θ_max$=15°, 30° and 60°). A fractional-step method was utilized with a generalized coordinate system. The flow patterns were displayed with instantaneous streamline, vorticity and pressure distributions. The main emphasis was placed on the lock-in range, which can be applied to flow feedback control. The numerical results clearly captured the lock-in phenomena. Several new phenomena in wakes were discovered by changing $S_f$ and $θ_max$. The relation between lift force and forcing frequency was also examined.
