Fluid-structure interaction of a flexible cantilever cylinder at low Reynolds numbers

We present a numerical study that investigates the fluid-structure interaction of a flexible cantilever cylinder with a uniform flow at low Reynolds numbers (Re). A fully coupled fluid-structure solver based on the three-dimensional Navier-Stokes equations and EulerBernoulli beam theory is employed to examine the coupled dynamics of the flexible cylinder. Of particular interest is to explore the possibility of flow-induced vibrations at laminar subcritical Re, i.e., no periodic vortex shedding, and assess the extent to which such a flexible cylindrical beam could sustain the vibrations at this Re regime. We find that when certain conditions are satisfied, the flexible cantilever cylinder experiences sustained vortex-induced vibrations (VIVs), with the frequency of the transverse oscillations matching the first-mode natural frequency of the cylinder. The range of the frequency match, known as the lock-in regime, is found to have a strong dependence on the Reynolds number Re, mass ratio m*, and reduced velocity U*. Unlike the steady wake behind a stationary rigid cylinder, the wake of the flexible cantilever cylinder is shown to become unstable at Reynolds numbers as low as Re = 22 when system parameters are within the lock-in regime. A combined VIV-galloping type instability is suggested as the mechanism behind the sustained unsteadiness in the wake and large-amplitude vibrations of the cylinder at laminar subcritical Re. These findings attempt to generalize our understanding of flowinduced vibrations in flexible cantilever structures and have a relevance to the development of novel bio-inspired flow-sensing devices.