FIV of tandem unequal-diameter flexible cylinders at different gap ratios

Abstract

Studies on the flow-induced vibration (FIV) of tandem unequal-diameter flexible cylinders in a broad gap ratio range are scarce. To reveal the effect of the gap ratio on and characterize the evolution features of the cylinder FIV with hydrodynamic interference, this paper experimentally investigates the FIV of two tandem flexible cylinders with unequal diameters at different gap ratios. Both cross-flow and in-line vibrations of the cylinders are permitted. Five gap ratios (i.e., T/d = 4, 6, 8, 10, and 12) are tested with the diameter ratio (d/D) set to be 0.5, where T is the center-to-center distance, d and D are the diameters of the upstream cylinder (UC) and downstream cylinder (DC), respectively. The effective cylinder length is 2.7 m, of which 1.2 m is submerged in water, and the aspect ratios of the UC and DC are 84.375 and 168.750, respectively. Based on the strains measured by the fiber bragg grating (FBG) sensors and the modal decomposition theory, detailed analyses of the displacement and frequency response are conducted. The FIV response of the UC resembles the vortex-induced vibration (VIV), whereas the DC undergoes wake-induced vibration (WIV). Increasing the gap ratio decreases the displacement amplitude of the DC but hardly affects the dominant frequency response. This study identifies three response regions (i.e., the VIV region, the transition region, and the WIV region) for the FIV response of the DC. The dual-cylinder interaction and dual-cylinder competition induce a coupled multi-physical vibration morphology of the cylinder. The “displacement-frequency co-variation” phenomenon of the DC and the “double-frequency lock-in” phenomenon of the UC are observed in the WIV region.