Flow interaction between a streamwise oscillating cylinder and a downstream stationary cylinder

Abstract

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 $$f_\mathrm{e}/f_\mathrm{s}$$ = 0–3.0, where $$f_\mathrm{e}$$ is the cylinder oscillation frequency and $$f_\mathrm{s}$$ 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 $$f_\mathrm{e}/f_\mathrm{s} > (f_\mathrm{e}/f_\mathrm{s})_\mathrm{c}$$ , where $$(f_\mathrm{e}/f_\mathrm{s})_\mathrm{c}$$ 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 $$0.5f_\mathrm{e}$$ . For 0.7–1.0 $$< f_\mathrm{e}/f_\mathrm{s} < (f_\mathrm{e}/f_\mathrm{s})_c$$ a complex vortex street that consists of two outer rows of vortices generated by the oscillating cylinder and two inner rows of vortices shed from the downstream stationary cylinder, which is referred to as AA-mode. For 0.3–0.6 $$< f_\mathrm{e}/f_\mathrm{s}<$$ 0.8–1.0, one single staggered vortex street (A-mode) is observed. It is also found that, when $$f_\mathrm{e}/f_\mathrm{s}$$ is near unity, the streamwise interaction of the two cylinders gives rise to the most energetic wake in the cross-stream direction, in terms of its maximum width, and the wake is AA-mode-like. The effects of other parameters such as the spacing between the two cylinders, Re and A/d on the flow pattern are also discussed in details. The observations are further compared to the stationary tandem cylinder cases.