In this paper, we present some experimental results about the physical effects of a cylinder’s streamwise oscillation motion on a downstream one in a tandem arrangement. The upstream cylinder undergoes a controlled simple harmonic oscillation at amplitudes A/d = 0.2–0.8, where d is the cylinder diameter, and the frequency ratio of fe/fsfe/fs = 0–3.0, where fefe is the cylinder oscillation frequency and fsfs is the natural frequency of vortex shedding from a single stationary cylinder. Under these conditions, the vortex shedding is locked to the controlled oscillation motion. Flow visualisation using the planar laser-induced fluorescence and qualitative measurements using hot-wire anemometry reveal three distinct flow regimes behind the downstream cylinder. For fe/fs>(fe/fs)cfe/fs>(fe/fs)c , where (fe/fs)c(fe/fs)c is a critical frequency ratio which depends on A/d and Reynolds number Re, a so-called SA-mode occurs. The upstream oscillating cylinder generates binary vortices symmetrically arranged about the centreline, each containing a pair of counter-rotating vortices, and the downstream cylinder sheds vortices alternately at 0.5fe0.5fe . For 0.7–1.
