The interference between two 5:1 rectangular cylinders in tandem is strongly affected by the gap width, which is relevant to parallel bridge decks and adjacent buildings. In the present study, three-dimensional large-eddy simulations are conducted to investigate the gap-width dependence of surface-flow and near-wake flow statistics, focusing on the mean velocity and Reynolds-stress distributions. Three gap widths are considered: G = 2D, 6D and 10D, where D is the cylinder height. The results show a transition of the overall flow structure from a reattachment pattern at G = 2D to a co-shedding pattern for G ≥ 6D. In addition to the global pattern transition, detailed surface-flow and near-wake statistics are presented to characterize the gap-width effects. For the upstream cylinder, the surface-flow structures and the Reynolds-stress features remain nearly unchanged with G, indicating weak sensitivity to gap width. In contrast, the downstream cylinder exhibits pronounced gap-width dependence: as G increases, a more developed recirculation bubble forms and flow recovery becomes faster, as evidenced by changes in the statistical distributions and surface-flow thickness. In the gap region, the mean velocity recovers more rapidly with increasing G, while the Reynolds-stress levels are reduced, indicating weakened turbulent activity sustained by the gap interactions. The Reynolds-stress distributions behind the downstream cylinder further reveal the effects of gap width on wake turbulence. These findings provide quantitative evidence of how gap width modifies surface-flow development and near-wake turbulence of tandem 5:1 rectangular cylinders.
Wang et al. (2026) studied this question.