This paper describes a numerical study of the two-dimensional and three-dimensional unsteady flow over two square cylinders arranged in an in-line configuration for Reynolds numbers from 40 to 1000 and a gap spacing of 4D, where D is the cross-sectional dimension of the cylinders. The effect of the cylinder spacing, in the range G = 0.3D to 12D, was also studied for selected Reynolds numbers, that is, Re = 130, 150 and 500. An incompressible finite volume code with a collocated grid arrangement was employed to carry out the flow simulations. Instantaneous and time-averaged and spanwise-averaged vorticity, pressure, and streamlines are computed and compared for different Reynolds numbers and gap spacings. The time averaged global quantities such as the Strouhal number, the mean and the RMS values of the drag force, the base suction pressure, the lift force and the pressure coefficient are also calculated and compared with the results of a single cylinder. Three major regimes are distinguished according to the normalized gap spacing between cylinders, that is, the single slender-body regime (G < 0.5), the reattach regime (G < 4) and co-shedding or binary vortex regime (G ≥4).
Hysteresis with different vortex patterns is observed in a certain range of the gap spacings and also for the onset of the vortex shedding. Copyright © 2011 John Wiley & Sons, Ltd.