2-DOF vortex-induced vibration of two rigidly-connected cylinders in parallel and tandem arrangements by a Cartesian grid method

A Cartesian grid fluid structure interaction (FSI) method with GPU acceleration is presented to simulate the vortex induced vibration (VIV) of two rigidly-connected cylinders in parallel and tandem arrangements. A highly efficient ghost cell method is adopted to treat arbitrary moving boundaries, and the motion equations of 2-DOF (Two-Degree-Of-Freedom) are solved by a RK4 (fourth-order Runge-Kutta) scheme. We investigated the variation pattern of the reduced velocity with the motion and dynamic responses, the phase switching, and the wake mode under three gap distances, two arrangements, and two mass ratios. It is found that the frequency lock-in occurs in a significantly larger reduced velocity for two cylinders compared with the single cylinder. The maximum response amplitudes of the displacement and force coefficients on two cylinders can be larger or smaller than those on the single cylinder depending on the tested conditions. Also, the soft lock-in phenomenon is observed for a small gap distance. Phase switching occurs several times for a large gap distance due to the wake effects. Furthermore, diverse wake modes are observed including merging, splitting, and impinging of the vortices. Especially, a new wake mode is found at a medium gap distance in the parallel arrangement.