On the oscillatory behavior of two pendulum-like tandem circular slender cylinders

This experimental study investigates the flow–structure interaction of two identical tandem slender cylinders subjected to cross-flow in a wind tunnel. The upstream and downstream cylinders were mounted in a pendulum-like configuration, allowing only transverse oscillation with matching natural frequencies, and were also tested in a fixed configuration for comparison purposes. All experiments were performed at spacing ratios ranging from 2.0 to 7.5, within a Reynolds number range of 16,000 to 22,000. Lateral displacements were recorded using a high-speed camera, and flow structures were analyzed using Particle Image Velocimetry (PIV). The primary objective was to correlate the observed flow dynamics from the PIV results with the oscillatory behavior of the cylinders. The results show that oscillations occurred for all investigated spacing ratios, with the most significant oscillations observed at a spacing ratio of 2.0 and a distinctive pattern with extreme amplitudes at a spacing ratio of 3.5. Cross-correlation of the Discrete Wavelet Transform of the vibration response reveals that the vibration of the downstream cylinder are influenced by the wake of the upstream cylinder, with its amplitude remaining enveloped within the wake of the upstream cylinder. The flow structure driving the oscillatory behavior indicates that the vibrations of the cylinders are predominantly governed by wake-induced vibration, except for the case of $L/D=3.5$, where a galloping-like instability was observed—a mechanism not previously reported for this configuration. The phenomenon described in this paper presents potential opportunity for energy harvesting applications.