Flow-induced vibration of two identical long flexible cylinders in a staggered arrangement

Abstract

The effects of spatially staggered arrangements on the flow-induced vibration (FIV) characteristics of two identical flexible cylinders were experimentally studied in a towing tank. Flexible cylinders with an aspect ratio of 350 and a mass ratio of 1.90 were free to vibrate in both the cross-flow (CF) direction and the in-line (IL) directions. During the experiments, the towing velocity varied from 0.05 m/s to 1.0 m/s with an interval of 0.05 m/s, corresponding to Reynolds numbers in the range of 800–16,000. A total of twelve different staggered arrangements for two-cylinder models were selected (S/D=2, 3, 4, 6 and T/D=4, 6, 8, where S and T are the center-to-center separation distances between the two cylinders in the CF and IL directions and D is the cylinder diameter). The experimental results show that the multi-mode vibrations of both flexible cylinders are influenced by the complicated flow interference. The Strouhal number St is sensitive to the CF spacing ratio (S/D) for both flexible cylinders. The displacements of the upstream cylinder are larger than those of the isolated cylinder due to proximity interference. The dynamic characteristics of the downstream cylinder are significantly affected by the wake interference. Wake-induced flutter can act on the downstream cylinder, leading to a large IL displacement and an elliptical motion trajectory. Two prominent frequency components are observed in the IL displacement response. The higher frequency component is related to the vortex shedding of the downstream cylinder. The lower frequency component, which approximates to the CF dominant frequency, is typical of wake-induced flutter.