Wake Topology of a Cylinder Undergoing Vortex-Induced Vibrations With Elliptic Trajectories

Wake vortex shedding topology of a cylinder undergoing vortex-induced vibrations (VIV) is investigated experimentally. Vibration measurements and flow visualization are utilized to study the connection between the cylinder response and the wake topology. The experiments were performed for two different orientations of the elliptic trajectories relative to the incoming flow at a fixed Reynolds number, moment of inertia ratio, mass ratio, and reduced velocity. Similar to the classical 2P regime, two counter-rotating vortex pairs are produced per oscillating cycle for both cases of elliptic trajectories examined here. However, significant changes in wake vortex dynamics are observed along the cylinder span. These changes include merging of vortices, which leads to shedding patterns similar to 2S and P + S modes downstream of the vortex formation region. The observed changes in vortex dynamics are accompanied by splitting of spanwise vortex filament and are attributed primarily to the changes in the local amplitude of vibrations along the span of the pivoted cylinder. It is shown that, being dependent on both the local amplitude of vibrations and vortex dynamics, the observed wake topology cannot be captured by the classical map of shedding regimes developed for VIV of one degree-of-freedom (DOF) cylinders.