Numerical study of aperiodic phenomena past two staggered rows of cylinders in cross-flow

The results of a numerical study of cross-flow past two staggered rows of cylinders are presented, at a fixed Reynolds number equal to 200. The upstream row comprised three and the downstream row four cylinders, for centre-to-centre pitch ratios in the streamwise direction, S/D, varying between 0 and 1. The distance between the centres of adjacent cylinders on the same row was equal to 3 cylinder diameters. The finite element method was employed for the solution. Significant fluctuations of the hydrodynamic forces, especially in the streamwise direction, were observed for all cylinders. Although for each longitudinal pitch ratio the variation of flow pattern with time appears almost chaotic, systematic attempt was made for the classification of the various flow regimes. A common characteristic of all longitudinal pitch ratios is the switch of wake width behind the cylinders of the downstream row. For S/D≤0.5 the formation of a large and symmetrical wake occurred intermittently past the downstream cylinders, with drastic impact on the entire flow domain. High shedding frequencies past the cylinders of the upstream row were detected for S/D≤0.5, which could reach two or even three times the shedding frequency past the cylinders of the downstream row.