The effects of parallel endplates on the secondary streaming of oscillating flows past a circular cylinder are studied by solving the Navier-Stokes equations numerically with Direct Numerical Simulation (DNS) method. A fractional-step scheme was implemented with the incorporation of the spectral method applied along the circumferential direction in solving a set of 2-D Poisson equations. The structure of the secondary streaming flows is presented with three-dimensional fluid particle traces (streak lines) and vorticity distributions. Unlike the traditional secondary streaming of two dimensional oscillating flows that exhibits 8 closed re-circulation zones (two in each quadrant), the mean secondary streaming flows in this study are three-dimensional without closed recirculation. The fluid particle traces show that there are three-dimensional spirals in each quadrant. Fluid particles near the endplates are attracted into the spirals toward the mid-plane of the two endplates. The trace trajectories in the flow domains never interest except at the stagnation points. The effects of cylinder aspect ratio, Keulegan-Carpenter number, and Stokes number on the secondary streaming patterns are also studied. The oscillatory drag and lift coefficients are also computed and discussed. The comparison of flow patterns obtained from this study with the results of experimental visualization shows qualitative agreement.

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