Published online by Cambridge University Press: 24 November 2011
A new type of flow-induced oscillation is reported for a tethered cylinder confined inside a Hele-Shaw cell (ratio of cylinder diameter to cell aperture, ) with its main axis perpendicular to the flow. This instability is studied numerically and experimentally as a function of the Reynolds number and of the density of the cylinder. This confinement-induced vibration (CIV) occurs above a critical Reynolds number much lower than for Bénard–Von Kármán vortex shedding behind a fixed cylinder in the same configuration (). For low values, CIV persists up to the highest value investigated (). For denser cylinders, these oscillations end abruptly above a second value of larger than and vortex-induced vibrations (VIV) of lower amplitude appear for . Close to the first threshold , the oscillation amplitude variation as and the lack of hysteresis demonstrate that the process is a supercritical Hopf bifurcation. Using forced oscillations, the transverse position of the cylinder is shown to satisfy a Van der Pol equation. The physical meaning of the stiffness, amplification and total mass coefficients of this equation are discussed from the variations of the pressure field.