Flow structures and aerodynamic forces of two tandem square cylinders at different sizes and spacing ratio

Using large eddy simulations, this study investigates the influence of the size ratio d/D and spacing ratio L/d on the flow field structure and aerodynamic interference mechanisms around two tandem square cylinders at a Reynolds number of Re=2×103, where d represents the size of the upstream square cylinder, D represents the size of the downstream square cylinder, and L represents the distance from the center of the upstream square cylinder to the windward face of the downstream square cylinder. A comprehensive analysis and discussion are conducted on the flow field structure, pressure distribution, force coefficients, power spectral density, and Strouhal number. The results reveal that the flow field structure and aerodynamic forces are highly sensitive to the size ratio and spacing ratio. Based on L/d, the flow around the two square cylinders can be primarily categorized into three regimes: the extended-body regime, reattachment regime, and co-shedding regime. Notably, when L/d is between the extended-body regime and the reattachment regime, two peaks in the fluctuating lift spectrum are observed. As d/D decreases, the critical L/d between the extended-body regime and reattachment regime decreases, while the critical L/d between the reattachment regime and co-shedding regime increases. Meanwhile, the width of the shear layer progressively narrows and becomes more tightly attached to the cylinder surface, leading to an increased vortex shedding frequency. Furthermore, a decrease in d/D also results in an increase in the time-averaged pressure coefficient Cp¯ and a decrease in the fluctuating pressure coefficient Cp′. When the regime transitions, changes in the flow structure are typically accompanied by abrupt changes in the aerodynamic parameters.