Insights to the Transition Regimes of Flow-Induced Vibration for Three Tandem Cylinders

Flow-induced vibration (FIV) is a complex phenomenon influenced by the interplay between flow dynamics and structural vibrations. Traditionally, it has been perceived as a threat to structures, but FIV has recently emerged as a promising method for highly efficient energy harvesting. This study focuses on the FIV of three tandem cylinders with varying spacing ratios (L/D) ranging from 1.3 to 4.0 with Reynold number Re of 5800-70000. Three distinct vibration states are identified: separated vortex-induced vibration (VIV) and wake-induced galloping (WIG) at L/D =1.3, continuous VIV-WIG at L/D =1.5, and partially separated VIV-WIG at L/D =2.0. To assess the coupling strength between the cylinders, the concept of vibration coherence is introduced and quantitatively analyzed. For L/D < 2, strong coherence is observed at the natural frequency ({f_{n), resulting in equal interference between the tandem cylinders across all reduced velocities. Conversely, for L/D >= 2, coherence becomes apparent at the vortex shedding frequency (f_v and 2 f_v), indicating linear interference with the reduced velocity. The transaction between the regimes is also accompanied with phase change. The establishment of these regimes, characterized by quantitative coherence, enhances our understanding of FIV in tandem cylinders, facilitating the efficient design of process industrial structures.