Numerical simulation of cross-flow around three equal diameter cylinders in an equilateral-triangular configuration at low Reynolds numbers

• Simulates the flow around three cylinders using the FVM with unstructured mesh.
• The biased flow is monostable, not bistable at low Re.
• The biased flow disappears at different spacing ratio for different Re.
• The characteristics of the interaction forces are dominated by pressure distribution.
• The influence mechanism of the fluctuating forces is related to spacing ratio.

Successful numerical simulation can reveal important flow characteristics and information which are extremely difficult to obtain experimentally. A two-dimensional finite volume method with unstructured mesh is used to simulate cross-flow around three equal diameter cylinders arranged in an equilateral-triangular interact manner for two incidence angles, α=0∘α=0∘ and α=180∘α=180∘. Special attention is paid on the variation of flow pattern characteristics among the cylinders, force characteristics and Strouhal number of each cylinder with nine spacing ratios (L/D) ranging from 1.5 to 7 at Re = 100 and 200, respectively. Three distinct flow patterns are revealed: (1) At α=0∘α=0∘ (inverted-T shaped arrangement), the biased flow generated behind the side by side downstream cylinders in small spacing ratio is not bistable but monostable, which means that the biased flow phenomenon once formed, it remained in this pattern. Furthermore, Reynolds number has significant impact on the spacing ratio where the biased pattern disappears, it disappears at L/D ≥ 2 for Re = 100 but L/D ≥ 3 for Re = 200. The biased phenomena results in a large difference in the mean and r-m-s drag coefficients for the side by side downstream cylinders. (2) An interaction force exists in the gap of the side by side cylinders that may be an attractive or a repulsive force dominated by the pressure distribution in the gap of the cylinders. (3) At small spacing ratio, the fluctuating forces on downstream cylinder(s) are very small and far less than that on a single cylinder due to the proximity effect. But at large spacing ratio, they are much larger than that on a single cylinder due to the fluctuation of the shear layer from the upstream cylinder(s). Furthermore, the spacing ratio where the effect of the fluctuation reaches its maximum is related to the Reynolds number, L/D=4L/D=4 at Re = 200 but L/D=5L/D=5 at Re = 100.