Flow around six in-line square cylinders

The flow around six in-line square cylinders has been studied numerically and experimentally for $0. 5\leq s/ d\leq 10. 0$ and $80\leq \mathit{Re}\leq 320$ , where $s$ is the surface-to-surface distance between two cylinders, $d$ is the size of the cylinder and $\mathit{Re}$ is the Reynolds number. The effect of spacing on the flow regimes is initially studied numerically at $\mathit{Re}= 100$ for which a synchronous flow regime is observed for $0. 5\leq s/ d\leq 1. 1$ , while quasi-periodic-I, quasi-periodic-II and chaotic regimes occur between $1. 2\leq s/ d\leq 1. 3$ , $1. 4\leq s/ d\leq 5. 0$ and $6. 0\leq s/ d\leq 10. 0$ , respectively. These regimes have been confirmed via particle-image-velocimetry-based experiments. A flow regime map is proposed as a function of spacing and Reynolds number. The flow is predominantly quasi-periodic-II or chaotic at higher Reynolds numbers. The quasi-periodic and chaotic nature of the flow is due to the wake interference effect of the upstream cylinders which becomes more severe at higher Reynolds numbers. The appearance of flow regimes is opposite to that for a row of cylinders. The Strouhal number for vortex shedding is the same for all the cylinders, especially for synchronous and quasi-periodic-I flow regimes. The mean drag ( ${C}_{Dmean} $ ) experienced by the cylinders is less than that for an isolated cylinder, irrespective of the spacing. The first cylinder is relatively insensitive to the presence of downstream cylinders and the ${C}_{Dmean} $ is almost constant at 1.2. The ${C}_{Dmean} $ for the second and third cylinders may be negative, with the value of ${C}_{Dmean} $ increasing monotonically with spacing. The changes in root mean square lift coefficient are consistent with changes in ${C}_{Dmean} $ . Interestingly, the instantaneous lift force can be larger than the instantaneous drag force on the cylinders. These results should help improve understanding of flow around multiple bluff bodies.