Flow-induced vibrations of circular cylinder in tandem arrangement with D-section cylinder at low Reynolds number

Elastically mounted circular cylinder in a fluid flow undergoes vortex-induced vibrations (VIV) and exhibits high amplitudes, however in a limited range of reduced velocity (UR). Studies have shown an introduction of asymmetry in the flow around the cylinder may lead to galloping, characterized by large amplitudes over a wide range of UR. Here, flow-induced vibrations of tandem arrangement of a D-section and a circular cylinder of equal diameter and density are studied computationally. The circular cylinder is placed in the wake-interference region. An in-house sharp-interface immersed boundary method has been used to solve for the fluid flow, while the rigid body dynamics of the cylinders are modeled through a linear spring-mass model. Over the range of UR considered (1≤UR≤15), the D-section cylinder shows both VIV and soft-galloping response characteristics. The excitation of galloping instability in the D-section is attributed to wake disruption by the circular cylinder. Oscillation frequencies of both cylinders are found to be in synchronization with the natural frequency of the structure once lock-in is attained. The circular cylinder's oscillations attain high amplitudes only when a transition in vortex shedding mode of the D-section cylinder is observed. The spectral characteristics of the forces and oscillations of the cylinders are studied, and overlapping VIV and galloping characteristics have been observed for both D-section and circular cylinders. In context of relevant literature, the wake-induced response of the cylinder is classified as galloping. The vorticity dynamics associated with the different regimes of response have been investigated.