An Experimental Study of the Near-Wake Structure of a Cylinder Undergoing Combined Translational and Rotational Oscillatory Motions

The experimental research reported here uses particle image velocimetry to extend the study of Nazarinia et al. [1], 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 translational and rotational velocities of the cylinder on the synchronization of the near-wake structures. 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 radian and a range of ratios between the translational and rotational velocities (V_R) = [0.25, 0.5, 1.0, 1.5]. In particular, it was found that varying the V_R value changed the near-wake structure. The results show that at lower values of V_R = 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 centerline and also synchronized with the combined oscillatory motion. As V_R 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 V_R = 0.5). Increasing VR further resulted in the appearance of unlocked-on regimes over the range of negative phase angles and a transition from 2S to P + S mode at the in-phase case There was transition back to the 2S with a further decrease of Φ. For a higher V_R the range of desynchronization increased.