Flow visualization behind a streamwise oscillating cylinder and a stationary cylinder in tandem arrangement

Abstract

Interference is investigated between a stationary cylinder wake and that of a downstream streamwise oscillating cylinder. Experiments were carried out in a water tunnel. A laser-induced fluorescence technique was used to visualize the flow structure behind two inline circular cylinders of identical diameter d. The downstream cylinder was forced to oscillate harmonically at the amplitude of 0.5 d and the frequency ratio f$_e$/f$_s$=1.8, where f$_e$ is the oscillation frequency of the downstream cylinder and f$_s$ is the vortex shedding frequency from an isolated stationary cylinder. The investigation was conducted for the cylinder center-to-center spacing L/d=2.5 3.5 and the 'two-cylinder shedding regime' at L/d > 3.5. At small L/d, the upstream cylinder does not appear to shed vortices; vortices are symmetrically formed behind the downstream cylinder as a result of interactions between the shear layers separated from the upstream cylinder and the oscillation of the downstream cylinder. This is drastically different from that behind two stationary cylinders at L/d > 3.5, where vortices are shed alternately from the downstream cylinder only. At L/d=4.5, both upstream and downstream cylinders shed vortices. This is true with or without the oscillation of the downstream cylinder. The flow structure is now totally different from that at L/d=3.5. The vortices are shed alternately from the upstream cylinder; a staggered spatial arrangement of vortices occurs behind the downstream cylinder.