Numerical simulations of flow past three circular cylinders in equilateral-triangular arrangements

Flow past three identical circular cylinders is numerically investigated using the immersed boundary method. The cylinders are arranged in an equilateral-triangle configuration with one cylinder placed upstream and the other two side-by-side downstream. The focus is on the effect of the spacing ratio L/D(=1.0-6.0), Reynolds number Re(=50-300) and three-dimensionality on the flow structures, hydrodynamic forces and Strouhal numbers, where L is the cylinder centre-to-centre spacing and D is the cylinder diameter. The fluid dynamics involved is highly sensitive to both Re and L/D, leading to nine distinct flow structures, namely single bluff-body flow, deflected flow, flip-flopping flow, steady symmetric flow, steady asymmetric flow, hybrid flow, anti-phase flow, in-phase flow and fully developed in-phase co-shedding flow. The time-mean drag and lift of each cylinder are more sensitive to L/D than Re while fluctuating forces are less sensitive to L/D than Re. The three-dimensionality of the flow affects the development of the wake patterns, changing the L/D ranges of different flow structures. A diagram of flow regimes, together with the contours of hydrodynamic forces, in the Re-L/D space, is given, providing physical insights into the complex interactions of the three cylinders.