Vortex shedding and lock-on of a circular cylinder in oscillatory flow

Abstract

An experimental study has been made of a circular cylinder in steady and oscillatory flow with non-zero mean velocity up to a Reynolds number of 40000. The results for the stationary cylinder are in close agreement with previously published data. Skin-friction measurements revealed the amplitude of fluctuation of the boundary layer for different angular locations. It has been universally accepted that bluff bodies shed vortices at their natural frequency of shedding (Strouhal frequency), or, when synchronized with an external unsteadiness, at the frequency of the disturbance or half of it, depending of the direction of the unsteadiness. Our findings, instead, indicate that the shedding frequency may vary smoothly with the driving frequency before locking on its subharmonic. Moreover, the present results indicate that, at the lowest frequency limit of lock-on, vortices are shed simultaneously on both sides of the model. A more traditional alternate pattern of vortex shedding is then recovered at higher driving frequencies.

Streamwise forced oscillations of circular and square cylinders

“…These studies report that the vortex shedding locks to a subharmonic of the forcing (vortex shedding at half the frequency of the forcing), particularly for forcing frequencies around twice f so , the shedding frequency from the unperturbed cylinder. Barbi et al 15 and Ongoren and Rockwell 18 also report that a synchronized symmetric wake structure, different from the classic von Kármán street, can be achieved depending on the amplitude and frequency of forcing, typically being found for high values of the frequency. Yokoi and Kamemoto 19 found a series of synchronized states, depending on forcing frequency, amplitude, and Reynolds number.…”

“…Others include, but are not limited to, transverse (cross-stream) oscillation, [1][2][3][4] rotational oscillation of the cylinder, [5][6][7][8] constant rotation of the cylinder, 9,10 and actuation by synthetic jets from the surface of the cylinder and base bleed. [11][12][13] Previous studies of streamwise forced oscillation [14][15][16][17] have investigated the synchronization between the forcing and the vortex shedding. These studies report that the vortex shedding locks to a subharmonic of the forcing (vortex shedding at half the frequency of the forcing), particularly for forcing frequencies around twice f so , the shedding frequency from the unperturbed cylinder.…”

Vortex shedding patterns, their competition, and chaos in flow past inline oscillating rectangular cylinders

“…This is indicative of subharmonic lockon [6,13], while the shedding is locked on to the oscillation of the cylinder in the S-III mode. Figure 8 shows the time signal of the vorticity ω at the monitor point C. In the absence of a mean flow, it is easy to visualise the alternate shedding of oppositely signed vortices when the cylinder is moving to and fro.…”

A Step Towards a Reduced Order Modelling of Flow Characterized by Wakes Using Proper Orthogonal Decomposition

“…As per suggestion by [7], the values of ∆U and f are selected so that the parameter ε = ∆U 2Π f D is around 0.2. This ensures a sufficiently large threshold window for lock-on to occur [8].…”