The laminar unsteady periodic flow motion found in the wake of rotationally oscillating circular cylinder is investigated numerically at a Reynolds number of 855 over a wide range of oscillation amplitude and fre- quency ratio. The governing Navier-Stokes equations are integrated to determine the flow field structure for large values of the time using an accurate spectral- finite difference method, but with the boundary vortic- ity calculated using global vorticity conditions rather than local finite-difference approximations. The lock- on phenomenon has been predicted and its effect on the flow hydrodynamics has been determined. The nu- merical scheme is verified by applying it to the special case of a steadily rotating cylinder (no forced oscilla- tions) and good qualitative agreements are found.
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