  The wake of a circular cylinder performing rotary oscillations is studied
using hydrodynamic tunnel experiments at $Re=100$. Two-dimensional particle
image velocimetry on the mid-plane perpendicular to the axis of cylinder is
used to characterize the spatial development of the flow and its stability
properties. The lock-in phenomenon that determines the boundaries between
regions of the forcing parameter space were the wake is globally unstable or
convectively unstable is scrutinized using the experimental data. A novel
method based on the analysis of power density spectra of the flow allows us to
give a detailed description of the forced wake, shedding light on the energy
distribution in the different frequency components and in particular on a
cascade-like mechanism evidenced for a high amplitude of the forcing
oscillation. In addition, a calculation of the drag from the velocity field is
performed, allowing us to relate the resulting force on the body to the wake
properties.
