Aperiodic phenomena in planar oscillatory flow past a square arrangement of four cylinders at low pitch ratios

A computational investigation of the viscous oscillating flow past four circular cylinders has been carried out, using the finite element technique. The cylinders were placed on the vertices of a square, two sides of which were perpendicular and two parallel to the oncoming flow, for pitch ratios, P/D, equal to 1.5 and 1.75. The frequency parameter was held constant and equal to 50, and the Keulegan-Carpenter number, KC, was ranging between 0.2 and 10. The presentation and discussion of results was focused for KC >= 4, where the flow is aperiodic in different oscillation cycles. In the aperiodic regime, computation over a large number of oscillation cycles was necessary for the resolution of the flow phenomena. An interesting feature when the flow is aperiodic is the existence of jumps in the time histories of the transverse forces, generated by a sudden switch in the flow pattern at two successive oscillation cycles. The mean transverse forces acting on the cylinders, the r.m.s. values of the in-line and transverse forces, together with the drag and inertia coefficients of the in-line force, are depicted in cumulative diagrams. These diagrams show the effect of the pitch ratio, for the entire range of Keulegan-Carpenter numbers examined. (C) 2012 Elsevier Ltd. All rights reserved.