Wake-induced vibrations of two elastically supported rigid cylinders in the tandem arrangement with large spacing ratios

An experimental study is conducted to investigate flow-induced vibrations of two elastically mounted circular cylinders in tandem at large spacing-to-diameter ratios of 7, 8, 9, 10, 12, and 18. The vibration responses and hydrodynamic forces of both cylinders are systematically analyzed, and distinct downstream regimes—wake-captured vibration (WCV), quasi-WCV, and wake-induced galloping—are identified. The response branch of the upstream cylinder is found to play a critical role in governing downstream behavior. In general, downstream vibration is suppressed when the upstream cylinder operates in the upper branch, with response levels up to 60% lower than those of an isolated cylinder, whereas it is enhanced in the lower branch, reaching amplitudes up to 40% higher than the isolated case at the smallest spacing. Even within the upper branch, different upstream response regimes produce markedly different downstream dynamics. Additional measurements of the mean inline force on a fixed downstream cylinder placed in the wake of a vibrating upstream cylinder show that a pure velocity-deficit interpretation is insufficient. In particular, the drag reduction is smaller when the upstream cylinder vibrates in the lower branch than when it is stationary, contradicting classical wake-deficit expectations. This indicates that changes in upstream vortex-shedding modes are likely to play an important complementary role in determining downstream excitation. Phase difference analysis further shows that vortex convection governs the phase relationship in the lower branch, while more complex coupling mechanisms dominate in the upper branch.