Flows past cylinders confined within ducts. Effects of the duct width

We present direct numerical simulations (DNS) of the flow past a circular cylinder confined in a rectangular duct at a Reynolds number of Re = 630, based on the bulk velocity and the cylinder diameter. The blockage ratio, defined as the ratio of the cylinder diameter to the duct height, is set at beta = 0.6. We find the topology of the wake is highly sensitive to the duct width, and suggest that care needs to be taken comparing results with experimental data sets. This is unlike the flow past an unconfined or lowly confined cylinder where the general topology remains relatively similar with duct width. Ducts of width 4.44, 10, and 20 times the cylinder diameter have been considered along with an infinite channel. For the smallest duct of 4.44 cylinder diameters, the effects of the side walls of the duct, or the end walls from the perspective of the cylinder, extend into the core region, where there are strong interactions between the influence from opposing sides. The wake formed is much more chaotic, with Kelvin-Helmholtz vortices observed to form in the shear layers off the cylinder. Hence leading to a shorter wake. With increases in the duct width to 10 cylinder diameters, the separation of the side walls lessens the interactions within the centre of the duct. The shear layers off the cylinder are less destabilised as a result, leading to the formation of a more classical Karman vortex street. The wake was also found to be longer as a result. However, the blockage effect was still evident at such a duct width. It was only for widths of 20 cylinder diameters or greater, a flow structurally similar to that in an infinite channel could be obtained, with asymmetric structures modulated along the span. Furthermore, the increases in the duct width beyond 10 cylinder diameters lead to decreases in the wake length.