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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When a pair of tandem cylinders is immersed in a flow the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. Assi, Bearman & Meneghini (J. Fluid Mech., vol. 661, 2010, pp. 365-401) concluded that the WIV excitation mechanism has its origin in the unsteady vortex-structure interaction encountered by the cylinder as it oscillates across the wake. In the present paper we investigate how the cylinder responds to that excitation, characterising the amplitude and frequency of response and its dependency on other parameters of the system. We introduce the concept of wake stiffness, a fluid dynamic effect that can be associated, to a first approximation, with a linear spring with stiffness proportional to Re and to the steady lift force occurring for staggered cylinders. By a series of experiments with a cylinder mounted on a base without springs we verify that such wake stiffness is not only strong enough to sustain oscillatory motion, but can also dominate over the structural stiffness of the system. We conclude that while unsteady vortex-structure interactions provide the energy input to sustain the vibrations, it is the wake stiffness phenomenon that defines the character of the WIV response.