The effect of imposed rotation on a slender elastically mounted circular cylinder free to oscillate transversely to the incident flow has been studied experimentally in a free-surface water channel. Rotation rate and direction are imposed to be proportional to either the cylinder’s transverse displacement or the cylinder’s transverse velocity to determine the effectiveness of these rotation laws to control the dynamics of the cylinder, either to reduce or to enhance oscillations. The former can be of interest for energy harvesting purposes whereas the latter can be useful to avoid unwanted oscillations. In all cases, non-dimensional mass and damping are fixed ($m^{\ast }=11.7$, $\unicode[STIX]{x1D701}=0.0043$) so the analysis is focused on the role of the rotation law and the reduced velocity. The Reynolds number based on the diameter of the cylinder ranges from 1500 to 10 000. Results are presented in terms of steady-state oscillation characterization (say, amplitude and frequency) and wake-pattern topology, which was obtained through digital particle image velocimetry. Both laws are able to either reduce or enhance oscillations, but they do it in a different way. A rotation law proportional to the cylinder’s displacement is more effective to enhance oscillations. For high enough actuation, a galloping-type response has been found, with a persistent growth of the amplitude of oscillations with the reduced velocity that shows a new desynchronized mode of vortex shedding. On the other hand, a rotation law proportional to the cylinder’s transverse velocity is more efficient to reduce oscillations. In this case only vortex-induced-type responses have been found. A quasi-steady theoretical model has been developed, which helps to explain why a galloping-type response may appear when rotation is proportional to cylinder displacement and is able to predict reasonably the amplitude of oscillations in those cases. The model also explains why a galloping-type response is not expected to occur when rotation is proportional to the cylinder’s velocity.
