  This paper numerically investigates the influence of a fixed downstream
control cylinder on the flow-induced vibration of an elastically-supported
primary cylinder. These two cylinders are situated in a tandem arrangement with
small dimensionless centre-to-centre spacing ($L/D$, $L$ is the intermediate
spacing, and $D$ is the cylinder diameter). The present two-dimensional (2D)
simulations are carried out in the low Reynolds number ($Re$) regime. The
primary focus of this study is to reveal the underlying flow physics behind the
transition from vortex-induced vibration to galloping in the response of the
primary cylinder due to the presence of another fixed downstream cylinder. Two
distinct flow field regimes, namely steady flow and alternate attachment
regimes, are observed for different $L/D$ and Re values. Depending on the
evolution of the near-field flow structures, four different wake patterns -
`2S', `2P', `2C', and `aperiodic' - are observed. The corresponding vibration
response of the upstream cylinder is characterized as interference galloping
and extended vortex-induced vibration. As the $L/D$ ratio increases, the lift
enhancement due to flow-induced vibration is seen to be weakened. The detailed
correlation between the force generation and the near-wake interactions is
investigated. The present findings will augment the understanding of vibration
reduction or flow-induced energy harvesting of tandem cylindrical structures.
