Flow over a cylinder subjected to combined translational and rotational oscillations

The experimental research reported here employs particle image velocimetry to extend the study of Nazarinia et al. (2009a), recording detailed vorticity fields in the near-wake of a circular cylinder undergoing combined translational and rotational oscillatory motions. The focus of the present study is to examine the effect of the ratio between the cross-stream translational and rotational velocities and frequencies on the synchronization of the near-wake structures for multiple phase differences between the two motions. The frequencies are fixed close to that of the natural frequency of vortex shedding. The results are presented for a fixed amplitude of rotational oscillation of 1 rad and a range of ratios between the translational and rotational velocities (VR)=[0.25,0.5,1.0,1.5](VR)=[0.25,0.5,1.0,1.5] and for a range of frequency ratios (FR)=[0.5,1.0,2.0](FR)=[0.5,1.0,2.0]. In particular, it was found that varying the VR value changed the near-wake structure. The results show that at the lower value of VR=0.25VR=0.25, for all of the phase differences examined, the vortices are shed in a single-row 2S mode aligned in the medial plane with a slight offset from the centreline and also synchronized with the combined oscillatory motion. As VR increases the vortex shedding mode changes from a 2S single-row to a 2S   double-row structure and eventually back to the single-row (at VR=0.5)VR=0.5). Increasing VR further resulted in the loss of lock-on over the range of negative phase angles and a transition from the 2S to P+S mode for the in-phase case. There was transition back to the 2S   wake mode with a further decrease in ΦΦ. For higher VR the range of desynchronization increased. In the second and third parts of this paper it is shown that the occurrence of unlocked wake flow as the phase angle is varied is greater when the frequency ratio between the imposed oscillatory motions and the natural vortex shedding frequency FRNFRN is higher than unity. The vortices are synchronized in the near-wake at FRNFRN values less than unity and unlocked when FRN>1.0FRN>1.0. In particular, the near-wake structures have also been shown to be synchronized for FR=0.5FR=0.5 and unlocked for FR=2.0FR=2.0.

► We investigate a cylinder in a free-stream with rotary and translating oscillations.
► The wake structure changes when the velocity ratio between the two motions change.
► Differences in phase between the oscillations also change the wake vortex structure.
► Depending on the velocity ratio and phase the flow may or may not be locked on.
► Lock-on also depends on the ratio of oscillation frequency to that of wake shedding.