Wake structure characteristics of three tandem circular cylinders at a low Reynolds number of 160

This paper reports the results of a numerical investigation into the flow around three isodiametric circular cylinders placed in an in-line configuration with identical spacing and the associated wake structures. The effect of spacing ratio (L/D, where L is the center-to-center spacing between two adjacent cylinders and D is the cylinder diameter) ranging from 1.5D to 10D with increment of 0.5D is examined at a low Reynolds number of 160. The results indicate that the wake structure experiences three evolutions as L/D increases, exhibiting four combined flow patterns: the overshoot, continuous reattachment-alternate reattachment (CR-AR), quasi-co-shedding (QCS), and co-shedding-co-shedding regimes. More than one frequency participates in the lift fluctuation for all three cylinders in the latter three patterns, signifying the wake interference. The evolution of the flow regime results in the variations of drag and lift coefficients, the alteration of pressure distribution around the cylinders' surface, and the switching of the phase lags of fluctuating lifts as well as the modification of vortex shedding characteristics, including the vortex formation location, wake width, and shedding frequencies. Particularly, the fluid forces and Strouhal number for the upstream and middle cylinders present a jump in the transition from the CR-AR to QCS regimes, while the fluid forces are reduced for the downstream cylinder. Furthermore, the transition from two layered vortices to the secondary vortex street is observed at 3.5 < L/D ≤ 6.5, where the downstream cylinder is sandwiched between the two shear layers detached from the middle cylinder, and two same-sign vortices in the same layer merge into a new vortex in the far wake.