Experimental study on the lock-in of an oscillating circular cylinder confined in a plane channel

Abstract

The results of an experimental study concerning wall boundary effects on the lock-in behavior of the shedding vortex of a circular cylinder, which is forced to oscillate transversely in a uniform flow at moderate Reynolds number Re = 2000,are presented in this paper. A height-adjustable flow channel is designed to study the boundary effect. Two sets of experiments are organized to study the lock-in behavior by changing the blockage ratio and oscillation amplitude separately. When the blockage ratio exceeds a critical value, the boundary effects are observed and the Strouhal number of the stationary cylinder increases non-linearly. As the blockage ratio increases, both the lower and upper limits of the lock-in region move to the lower value of excitation frequency, and the extent of the lock-in region decreases first and then increases. The boundary changes the effects of oscillation amplitude on lock-in behavior. When free from the boundary effect, as the oscillation amplitude increases, a V-shaped boundary of the lock-in region is observed, and the extent of the lock-in region increases monotonically. In comparison, under the influence of the boundary, the extent of the lock-in region first increases with the increase of the oscillation amplitude, and then decreases with the increase of the oscillation amplitude. Hysteresis loops are first observed near the lower limits of the lock-in region for some ranges of blockage ratio and oscillation amplitude in a forced-vibration system, and the extent of the hysteresis loop is affected by the two parameters.