Two-degree-of-freedom vortex-induced vibrations of a spring-mounted rigid cylinder with low mass ratio

This work studies the vortex-induced vibrations of an elastically mounted rigid cylinder able to move in-line and transverse to the flow, with equal mass ratio and natural frequencies in both directions. A compact and simple set-up using torsion springs was designed for this purpose. Although the mass ratio is close to 1, the mass-damping parameter α is high enough so that the system exhibits a two-branch type response. Consequently, the ‘super-upper’ branch is not observed, and the peak transverse response is unaffected compared to transverse-only oscillations. We also note that measurement points associated with a ‘2S’ mode of vortex formation are found in the ‘2P’ area of the map of vortex regimes. This discrepancy is discussed in light of the influence of streamwise motion on the critical curve separating the ‘2S’ and ‘2P’ regimes.