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The geometry which has been the subject of most research on vibrating bluff bodies is a circular cylinder. For a circular cylinder, the vortex-shedding frequency fv and the Strouhal frequency Is are generally the same. However, as.fs comes closer to the natural frequency of vibration f, of the cylinder, the vortex-shedding frequency f, locks in to the natural frequency of vibration of the cylinder f,, and the Strouhal frequency .fs is lost. This means that both the cylinder and the wake have the same characteristic frequency. Moreover, there is a narrow band of frequency around the Strouhal frequency, where the vortex-shedding frequency fv corresponds to the natural frequency of vibration of the cylinder f, and not to the Strouhal frequency .fs. Under these conditions, if the damping of the cylinder is less, self-excited resonant vibration may develop in the cylinder. For a narrow band of fluid velocity, the amplitude of vibration can be of the order of 1 D [140]. The plane of vibration of the cylinder is generally perpendicular to the flow, although under certain conditions the cylinder may also vibrate in-line with the flow.