Frequency lock-in mechanism in the presence of blockage effects

Abstract

This study reveals the blockage effects on vortex shedding, the lock-in mechanism of a forced oscillating cylinder, and the coupling effects of blockage and oscillation. The wind tunnel experiments and large eddy simulations were conducted at a Reynolds number of 2000, encompassing a range of blockage ratios from 0.15 to 0.5. The cylinder is subjected to forced harmonic oscillations in a direction perpendicular to the incoming flow to investigate the frequency lock-in mechanism. The research findings demonstrate that both blockage and forced vibration significantly influence the dynamics of vortex shedding by altering the development of the shear layer instability (SLI). For a fixed cylinder, high blockage effectively suppresses the growth of transverse disturbances, promoting a stable maintenance of the shear layer (SL). In high blockage, the flow deceleration induces the separation of the boundary layer from the sidewalls and the subsequent constriction of the mainstream toward the channel centerline, consequently leading to an increase in the Strouhal number. For the case of a fixed blockage ratio, forced vibration alters the flow supplement in the near-SL region. During the unlocked phase, a significant occurrence of backflow is observed near the SL, which promotes the development of SLI. However, proper vibration induces the transverse flow supplement in the near-SL region, which balance the amount carried away by mainstream, thereby suppressing the backflow. The coupling effects of blockage and vibration are evident in the shift of the lock-in region and the triggering of hysteresis, both of which are explained by the proposed mechanisms of blockage and lock-in.