Hydrodynamic characteristics of a confined circular cylinder in cross-flows

The hydrodynamics of a circular cylinder in confined flows have great significance to fundamental fluid mechanics and engineering applications. However, a major gap of knowledge exists regarding the impact of blockage on the hydrodynamic properties of the cylinder including the fluctuating lift coefficient (C-L'), mean drag coefficient (C-D) and Strouhal number (St). To fill the knowledge gap, three-dimensional numerical simulations are conducted at blockage ratios up to beta = 1/2 (where beta = D/H; D and H are the cylinder diameter and channel height respectively) and Reynolds numbers from Re = 300 to 30,000. The present study reveals that confinement causes a significant increase of C-D and St, whereas different behaviours of C-L' are observed in laminar and turbulent flow regimes. Further, a reverse Karman vortex street is observed at all blockage ratios between 1/6 and 1/2 for Reynolds numbers less than 1,000. The invert location, where the vortex trajectories originating from the top and bottom of the cylinder intersect in the wake, depends on the Reynolds number and the blockage ratio. At Reynolds numbers above 1,000, the reverse Karman vortex street does not occur, and the vortices organise themselves in a single row with an alternate arrangement in the wake.