Cross-flow vibrations of two identical elastically mounted cylinders in tandem arrangement using wind tunnel experiment

In this paper, cross-flow vibrations of two identical elastically mounted circular cylinders in tandem arrangement have been investigated in a wind channel at Reynolds number Re = 4000-42000. The spacing ratios S/D are from 1.2 to 8.0. The vibration amplitude, oscillation frequency f(o) and vortex shedding frequency f(s) are investigated. Based on the vibration characteristics, three regimes can be classified. In Regime I (S/D <= 1.5), the upstream cylinder experiences galloping vibration, while the downstream cylinder undergoes fluctuating with multiple peaks; In Regime II (1.5 < S/D <= 3.0), two separated vibration regions can be detected for both cylinders. In Regime III (3.0 < S/D <= 8.0), the upstream cylinder presents a typical vortex-induced vibration (VIV), while the downstream cylinder displays two regions including a VIV and wake-induced galloping (WIG). A hysteresis phenomenon can be observed at S/D = 1.5 and 2.0 for different initial test conditions ('from rest' and 'increasing velocity'). The effect of the initial states (fixed or elastically mounted) of the neighbouring cylinder on the other cylinder is also significant. The f(s) can be locked to the multiple harmonics of natural frequency f(n), while the dominant f(o) can only be locked to the first harmonic of f(n).