Decomposition of fluid forcing and phase synchronisation for in-line vortex-induced vibration of a circular cylinder

We present a decomposition of the streamwise fluid force for in-line vortex-induced vibration (VIV) to provide insight into how the wake drag acts as a driving force in fluid-structure interaction. This force decomposition is an extension of that proposed in the recent work of Konstantinidis et al. (J. Fluid Mech., vol. 907, 2021, p. A34), and is applied to and validated by our experiments examining a circular cylinder freely vibrating in line with the free stream. It is revealed from the decomposition and linear analysis that two regimes of significant vibration are in phase synchronisation, while they are separated by a desynchronised regime marked by competition between non-stationary frequency responses of the cylinder vibration and the vortex shedding. Of interest, such a near-resonance desynchronisation regime is not seen in the transverse vibration case.