Dynamics of vortex lock-on in a perturbed cylinder wake

The dynamics of vortex lock-on, downstream of a circular cylinder in an oscillatory flow with nonzero mean velocity, is investigated by applying a time-resolved particle image velocimetry technique at the Reynolds number 360. Since the lock-on occurs when the near wake behind a cylinder is perturbed at twice the natural shedding frequency, we interrogate the wake regions of mean recirculation and vortex formation, and analyze the dynamic behavior of the shed vortices, the coherent structure, the phases of vortex evolution, and the Reynolds stress fields. It is shown that due to vortex lock-on the mean recirculation and vortex formation regions are considerably reduced, which is consistent with previous studies. Besides, the kinetic energy in the near wake is more concentrated near the cylinder base. A dramatic change is demonstrated in the trajectory of the shed vortices, which is evaluated from a hybrid method based on complex eigenvalue and vortex centroid. A novel method to identify the phases of the vortex evolution is proposed in accordance with the distribution of the coherent Reynolds shear stress. Then, it is shown that two flow fields in the natural shedding and lock-on states have a phase difference of about π/4. As a result of vortex lock-on, the distributions of the Reynolds stresses exhibit not only a stronger Kármán vortex, but also a more synchronized pattern with a shortened wake region. From the streamwise force balance on the wake bubble, it is noted that an outstanding decrease (a reversal) of the shear stress by the lock-on results in an increase of the drag force and that the energy transfer from random component to coherent component decreases the three dimensionality of the cylinder wake, inducing the above-mentioned shedding of stronger Kármán vortices.