On Wake Interference in the Flow around Two Circular Cylinders: Direct Stability Analysis and Flow-Induced Vibrations

Abstract

The flow around two identical circular cylinders, arranged in configurations where one of the cylinders is immersed in the wake of the other, is studied using numerical simulations. Two aspects of such flows were considered. The first is the stability of nominally two-dimensional time-periodic wakes to three-dimensional perturbations. We investigated flows around tandem and staggered arrangements with diverse centre-to-centre distances. Direct stability analysis and numerical simulations were employed, and the results are compared to those obtained for the flow around an isolated cylinder. The onsets of the three-dimensional instabilities were calculated and the unstable modes are fully described. In addition, we assess the nonlinear character of the bifurcations and physical mechanisms are proposed to explain the instabilities. The second aspect considered in this thesis is the flowinduced vibration experienced by a rigid cylinder when it is mounted on an elastic base and immersed in the wake of a fixed identical cylinder. Tandem arrangements with centre-to-centre distances varying from 1.5 to 8 cylinder diameters were tested. The flow was simulated using an Arbitrary Lagrangian-Eulerian approach that coupled the solution of the structure equations with that of the flow. Two- and three- dimensional simulations were performed to assess the mutual influence between the three-dimensional flow structures in the wake and the motion of the cylinder. The response of the downstream cylinder is compared to that of an elastically-mounted rigid isolated cylinder. Based on the simulation results we propose physical mechanisms to explain the origin of the excitation.