Three-dimensional flow past a line of cylinders: Influence of spanwise length

Although previous studies have demonstrated the influence of spanwise length on the flow past an isolated cylinder, the effect of spanwise length on the flow past multiple cylinders has not been investigated, and no guidance exists on the appropriate choice of spanwise length for such configurations in numerical simulations. This gap motivates the present study, which employs three-dimensional direct numerical simulations, for the first time, to examine the influence of spanwise length on the flow past a line of 2–11 cylinders at a Reynolds number of 200 (Red=U∞d/υ, where U∞ is the velocity upstream of the cylinder, d is the cylinder diameter, and υ is the fluid viscosity). The spanwise length Lz is varied in the range of 1–48d. Significant variations in flow and force characteristics are observed as the spanwise length changes within the range. Varying the spanwise length can significantly alter wake transitions along the line of cylinders. While three-dimensionality in the system is entirely suppressed for Lz≤ 2d, fully developed three-dimensional flow features, including mode A and vortex dislocations, are captured for Lz≥ 16d. This critical length of 16d is found to be independent of the number of cylinders. Furthermore, applying periodic boundary conditions in the spanwise direction with a spanwise length that is not an integer multiple of the characteristic length scale of mode A structures (4d for an isolated cylinder) can introduce artificial disturbances into the system, leading to unphysical flow irregularities for a shorter spanwise domain (Lz< 16d) where natural vortex dislocations are suppressed. Based on numerical convergence and computational cost considerations, a spanwise length of at least 16d is recommended for accurately studying the flow past a line of cylinders at Red≈ 200.