Wake and suppression of flow-induced vibration of a circular cylinder

This experimental study examines the wake flow and the flow-induced vibration of an elastically supported circular cylinder (referred to as test cylinder, of diameter D), and then the suppression of the flow-induced vibration by a control cylinder (of diameter 0.5D) installed behind the circular test cylinder is investigated at Reynolds number Re = 1.4 × 104-3.2 × 104 corresponding to the reduced velocity Ur = 0.4-22. The streamwise and lateral separations of the control cylinder from the test cylinder center are varied as 0.2D - 1.1D and 0 - 1.7D, respectively. The vortex trail encompassing the vortex generation to the disappearance in the wake of the test cylinder is identified by means of wake energy distributions obtained from power spectra of fluctuating streamwise velocities. Based on the energy distributions, the control cylinder positions are determined to suppress the flow-induced vibration. The process of vortex 'Generation → Growth → Disappearance' in the wake of a vibrating cylinder occurs closer to the cylinder compared to that of a non-vibrating cylinder. The lateral separation between the two rows of vortices for a vibrating cylinder is found to be smaller. The controlled vibration responses of the cylinder are classified into four patterns (I, II, III, and IV) depending on the control cylinder position. Patterns I and II, both self-limited amplitude vibrations, are characterized by the occurrence of vibrations at a smaller and a larger Ur, respectively, compared to the non-controlled counterpart. Pattern III represents an increasing vibration amplitude with an increase in Ur, resembling a galloping vibration. Pattern IV features vibration suppression resulting from the suppression of the Karman street in the wake. This regime is divided into two subregimes, IV-A and IV-B. While the interaction between the gap flow and freestream side shear layer of the test cylinder succeeds in the suppression of the Karman street in pattern IV-A, that between the gap shear layer and the control cylinder gives rise to the suppression of the Karman street in pattern IV-B.