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This study presents a novel investigation into the vortex dynamics of flow around a near-wall rectangular cylinder based on direct numerical simulation at Re=1000, marking the first in-depth exploration of these phenomena. By varying aspect ratios (L/D = 5, 10, 15) and gap ratios (G/D = 0. 1, 0. 3, 0. 9), the study reveals the vortex dynamics influenced by the near-wall effect, considering the incoming laminar boundary layer flow. Both L/D and G/D significantly influence vortex dynamics, leading to behaviours not observed in previous bluff body flows. As G/D increases, the streamwise scale of the upper leading edge (ULE) recirculation grows, delaying flow reattachment. At smaller G/D, lower leading edge (LLE) recirculation is suppressed, with upper Kelvin–Helmholtz vortices merging to form the ULE vortex, followed by instability, differing from conventional flow dynamics. Larger G/D promotes the formation of an LLE shear layer. An intriguing finding at L/D = 5 and G/D = 0. 1 is the backward flow of fluid from the downstream region to the upper side of the cylinder. At G/D = 0. 3, double-trailing-edge vortices emerge for larger L/D, with two distinct flow behaviours associated with two interactions between gap flow and wall recirculation. These interactions lead to different multiple flow separations. For G/D = 0. 9, the secondary vortex (SV) from the plate wall induces the formation of a tertiary vortex from the lower side of the cylinder. Double-SVs are observed at L/D = 5. Frequency locking is observed in most cases, but is suppressed at L/D = 10 and G/D = 0. 9, where competing shedding modes lead to two distinct evolutions of the SV.
Li et al. (Tue,) studied this question.