Interactions of lock-on wake behind side-by-side cylinders of unequal diameter at Reynolds number 600

Abstract

The flow characteristics, mutual interactions and downstream evolutions of the wakes behind the side-by-side cylinders of unequal diameter are illustrated by flow visualization and Laser Doppler Anemometry. All the experiments are conducted in a recirculating water channel at Reynolds number 600. The gap ratio is 0.75 and the diameter ratio is 2.0. Periodic excitations are applied to the large cylinder with constant excitation amplitude. Within the excitation frequency range studied, the wake behind the large cylinder experiences the primary and the one third sub-harmonics lock-on; whereas the wakes behind the small cylinder are not locked-on. For the wake behind the large cylinder experiencing the primary lock-on, the gap flow becomes unbiased and the gap vortices are observed to interact and shed alternately into the wake behind each cylinder in synchronization with the excitation frequency (the natural vortex shedding frequency of large cylinder). The synchronized flow patterns exist only in the near wake region; in the downstream region, only one wider wake dominated by the excitation frequency is detected. While the wake behind large cylinder is locked-on at the one third sub-harmonics of the excitation frequency, the gap vortices are observed to entrain alternately into the wake behind each cylinder in synchronization with the one third of the excitation frequency (e.g., natural vortex shedding frequency of large cylinder). The synchronized flow patterns exist only in the near wake region; in the downstream region, the wake is relatively weak, less organized and turbulent. For both lock-on cases, the near and the far wake structures change suddenly within a short distance.