On the flip-flopping phenomenon of two side-by-side circular cylinders at a high subcritical Reynolds number of 1.4 x 105

To investigate the flow characteristics of the biased gap flow and the flip-flopping phenomenon at high Reynolds numbers, large-eddy simulation is adopted to simulate the crossflow around two side-by-side circular cylinders at Re = 1.4 x 10(5). The center-to-center pitch ratio P/D varies from 1.1 to 4. Three flow patterns, i.e., single bluff body regime, biased gap flow regime, and coupled vortex regime, have been successfully simulated. A tri-stable flow pattern is found at P/D = 1.1; namely, a single bluff body regime and a biased gap flow regime deflected to the upper or lower cylinder occur spontaneously and intermittently. In contrast to the results obtained at low Reynolds numbers, two distinct biased gap flow structures are observed at P/D = 1.2 and 1.5, respectively. For P/D = 1.2, the gap-side shear layer separated from the narrow wake cylinder further reattaches to the cylinder; namely, a series of separation bubbles would develop along the gap-side surface of the cylinder. These unsteady separation bubbles induce the inward-directed lift force on the cylinder. For P/D = 1.5, the gap-side shear layer separates directly without further reattachment, and both cylinders experience outward lift forces. It is revealed that the flip-flopping of the biased gap flow is associated with the interaction of the gap vortices and the vortex shed from the outer-side shear layer of the wide wake cylinder. Furthermore, the flip-flopping phenomenon arises initially at one particular cross section of the cylinder, rather than occurring simultaneously along the spanwise direction of the cylinder.