Flow-induced vibration of filaments attached to two side-by-side cylinders

Flow-induced vibration of flexible filaments attached to two side-by-side circular cylinders in the laminar regime is studied for a range of reduced speed (U^*). U^* is defined as the freestream speed of the flow nondimensionalized by the fundamental natural frequency of the filaments and cylinder diameter. Dominant modes in the response are identified using proper orthogonal decomposition. The flow and response of the filaments are compared to that of a single cylinder with a flexible filament, from an earlier study. [M. Furquan and S. Mittal, “Multiple lock-ins in vortex-induced vibration of a filament,” J. Fluid Mech. 916, R1 (2021).] The filaments vibrate with small amplitude close to the frequency of vortex shedding from the stationary structure, at low U^*. This desynchronization regime is followed by a lock-in with the first Euler-Bernoulli mode beyond a certain U^*. Further increase in reduced speed results into a region of aperiodic oscillations followed by a lock-in with the second Euler-Bernoulli mode. The aperiodic region is in lieu of the second desynchronization range observed for a single cylinder with filament. Frequency spectra of the former contain significant contributions from both vortex shedding from the stationary structure, as well as the first structural mode. The two filaments oscillate largely in-phase and with equal amplitudes up to the onset of second lock-in. Vortex shedding from the two cylinders is synchronized and in-phase. The onset of lock-in with the second structural mode marks the advent of a symmetry-breaking bifurcation that leads to a bias in the mean configuration of the filaments as well as the difference in their amplitude of vibration and a disorganized wake.