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The mechanism of wake-induced vibrations (WIV) of a pair of cylinders in a tandem arrangement is investigated by experiments. A typical WIV response is characterized by a build-up of amplitude persisting to high reduced velocities; this is different from a typical vortex-induced vibration (VIV) response, which occurs in a limited resonance range. We suggest that WIV of the downstream cylinder is excited by the unsteady vortex-structure interactions between the body and the upstream wake. Coherent vortices interfering with the downstream cylinder induce fluctuations in the fluid force that are not synchronized with the motion. A favourable phase lag between the displacement and the fluid force guarantees that a positive energy transfer from the flow to the structure sustains the oscillations. If the unsteady vortices are removed from the wake of the upstream body then WIV will not be excited. An experiment performed in a steady shear flow turned out to be central to the understanding of the origin of the fluid forces acting on the downstream cylinder.Key words: flow-structure interactions, vortex flows, wakes/jets IntroductionGreat strides have been made in understanding the mechanisms involved in vortexinduced vibration (VIV) of an isolated circular cylinder free to vibrate transverse and/or in-line to a fluid flow. Progress has been reviewed by Sarpkaya (1979Sarpkaya ( , 2004, Bearman (1984), Parkinson (1989), Blevins (1990) and Williamson & Govardhan (2004). With the development of offshore oil fields and the deployment of riser pipes, much of the research related to cylinder response focused on conditions with low mass ratio m * and damping ζ . Here, m * is the ratio of mass per unit length of the structure, m, to fluid displaced, ρπD 2 /4, where ρ is fluid density, D is cylinder diameter and ζ is structural damping expressed as a fraction of critical damping. This research revealed a number of extremely interesting phenomena associated with VIV of isolated cylinders.However, there are many flow-induced vibration (FIV) problems that involve two or more cylinders in configurations where the flow field of one cylinder influences

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Direct stability analysis and numerical simulations have been employed to identify and characterize secondary instabilities in the wake of the flow around two identical circular cylinders in tandem arrangements. The centre-to-centre separation was varied from 1.2 to 10 cylinder diameters. Four distinct regimes were identified and salient cases chosen to represent the different scenarios observed, and for each configuration detailed results are presented and compared to those obtained for a flow around an isolated cylinder. It was observed that the early stages of the wake transition changes significantly if the separation is smaller than the drag inversion spacing. The onset of the three-dimensional instabilities were calculated and the unstable modes are fully described. In addition, we assessed the nonlinear character of the bifurcations and physical mechanisms are proposed to explain the instabilities. The dependence of the critical Reynolds number on the centre-to-centre separation is also discussed.

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