Experiments on flow-induced vibration of four flexible cylinders with large aspect ratio in a square configuration

Abstract

A system consisting of four cylinders positioned in the quadrilateral arrangement represents the prevalent and fundamental constituent within pipe bundles and tube banks in numerous engineering scenarios. This research presents a laboratorial study on flow-induced vibration (FIV) exhibited by four flexible cylinders positioned within a square geometric arrangement. The uniform flows were produced by a carriage on the top of a towing tank. Considering flow velocities and structural parameters, Reynolds number could reach 16000. The spacing between the cylindrical centres was selected as 6.0 D (D denotes the outer diameter of cylinders), and two typical incidence angles 0° and 45° were examined. For each flexible cylinder model, strain gages at seven distinct measuring positions were utilized to collect the oscillation data in cross-flow (CF) and in-line (IL) directions. Principal vibration modes, response frequencies, response amplitudes and motion trajectories were presented to understand the FIV features. For incidence angles 0°, considering the four flexible cylinders as two sets of tandem cylinders arranged in parallel is not appropriate due to the proximate effect. The behaviours of the two upstream cylinders resembles those of the pair of cylinders positioned in a side-by-side configuration, whereas behaviours for the downstream cylinders are influenced by wake interference generated from leading cylinders. IL response frequencies decrease to the values close to the CF response frequencies, resulting in the elliptical-shaped motion trajectories with large IL displacements. For incidence angles 45°, the upstream cylinder displays characteristics akin to those of a solitary cylinder experiencing vortex-induced vibration (VIV). Reactions for the central pair of cylinders result from the amalgamation of VIV and wake-induced flutter. While, reactions observed for downstream cylinder are characterized by low frequencies, significant IL displacements and more chaotic trajectories.