Flow around two elastically-mounted cylinders with different diameters in tandem and staggered configurations in the subcritical Reynolds number regime

Flow around two cylinders with different diameters undergoing Flow-Induced Vibrations (FIV) in the subcritical flow regime is investigated using two-dimensional Unsteady Reynolds Averaged Navier-Stokes (URANS) approach. Physical parameters of the system are chosen to represent the free spanning pipelines laid in proximity. The two cylinders are initially placed at various tandem and staggered positions with one in the wake of the other, and subject to steady current flows. The two cylinders are free to respond in both in-line and transverse directions. The investigated Reynolds numbers (Re) are Re1=1.4×105 based on the diameter of the larger cylinder and Re2=1.15×105 based on the diameter of the smaller cylinder. A parametric study investigating the effects of relative spacing of the cylinders on the vibration response of the system is performed. First and second order statistics of the flow, frequency domain analysis and flow field visualizations are used to characterize the dynamic behavior of the system. It is found that the motion trajectories of the downstream cylinder show a qualitative difference depending upon whether it is in tandem with the upstream cylinder or in the wake with a transverse offset. A large amplification of the in-line response is observed in the positions with a transverse offset. The vibration response of the upstream cylinder is affected by the presence of the downstream cylinder only when the horizontal center-to-center distance is small (L/D1=2.06 where D1 is the diameter of the larger cylinder) and is largely unaffected when the horizontal distance is increased to L/D1=3.22. •Flow around two cylinders with different diameters and free to vibrate in two degrees-of-freedom was investigated using CFD simulations.•Different horizontal and vertical offsets of the downstream cylinder were simulated.•Hydrodynamic lift and drag coefficients were mapped with respect to the initial position of the downstream cylinder. In addition to force coefficients, the vibration amplitude responses in cross-flow and in-line directions were mapped for both cylinders.•Three distinct response zones have been identified based on the vibration responses and configuration parameters.•In the high response zone at intermediate vertical offsets the in-line vibration amplitudes of the downstream cylinder are amplified significantly compared to those in the tandem configuration or outside of the wake interference range.