Vortex-induced vibration and wake interference of two rigidly connected tandem cylinders

This paper numerically investigated the vortex-induced vibration of two rigidly connected tandem cylinders with streamwise spacing ranging from 1.5D to 4.5D. The two identical cylinders possess a mass ratio of 2 and a damping ratio of 0.001. Simulations are conducted at a low Reynolds number of 100 over a reduced velocity range of 2-18. Five wake interference modes are identified in terms of the evolution of vortex shedding and shear layers at different Ur and L/D, including the overshoot mode, continuous reattachment mode, alternate reattachment (AR) mode, quasi-co-shedding (QCS) mode, and co-shedding (CS) mode. Furthermore, the AR mode can be divided into vortex merging (ARM) mode and vortex shedding (ARS) mode on the basis of whether the shed vortex from the upstream cylinder merges with the shed vortex from the downstream cylinder. The wake transition from two-layered vortices to the secondary vortex street is observed when L/D = 3.5 at Ur = 5, and when L/D = 4.5 in Ur range of 5-8, where the tandem cylinders experience the larger cross-flow vibration amplitudes with the occurrence of QCS or CS mode.