The flow characteristics, mutual interactions and downstream evolutions of the wakes behind the sideby-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 gapratio is 0.75 and the diameter ratio is 2.0. Periodic excitations are applied to the large cylinder with constantexcitation amplitude. Within the excitation frequency range studied, the wake behind the large cylinderexperiences the primary and the one third sub-harmonics lock-on; whereas the wakes behind thesmall cylinder are not locked-on. For the wake behind the large cylinder experiencing the primary lockon,the gap flow becomes unbiased and the gap vortices are observed to interact and shed alternately intothe wake behind each cylinder in synchronization with the excitation frequency (the natural vortex sheddingfrequency of large cylinder). The synchronized flow patterns exist only in the near wake region; inthe downstream region, only one wider wake dominated by the excitation frequency is detected. Whilethe 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 synchronizationwith the one third of the excitation frequency (e.g., natural vortex shedding frequency of largecylinder). 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 farwake structures change suddenly within a short distance
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