Flow cross around four circular cylinders arrangement near a plane wall could be found in many engineering application such as subsea pipeline, shell and tube heat exchangers, and tube banks, etc. When flow crosses the bodies, it would form a particular flow pattern according to the bodies arrangement. The objective of this research is to study numerically the effect of gap (G) on flow characteristics around four cylinders placed near a plane wall. Gap ratio (G/D) is defined as ratio between a distance of the wall to a surface of the bottom of the lower cylinders (G) and the cylinder diameter (D). It is varied of 0.2, 0.3, 0.5, 0.7, 1.0 and 1.5 with critical spacing ratio L/D (where L is a distance of cent re of cylinders) is fixed at 2.7. A numerical simulation with two dimensional Unsteady Reynolds Averaged Navier-Stokes equations and k-ω SST turbulence model is solved with a finite volume method has been applied to investigate this problem. To analyze flow characteristics around the cylinders is used FLUENT 6.3.26 software, and it is performed with a Reynolds number 53000 based on a single circular cylinder diameter and free stream velocity. The numerical results show that an increasing of the gap ratios causes the changing of flow patterns around of each cylinder. The stagnation point of the lower-upstream cylinder (cylinder-1) moves to the front-side of the cylinder, while it is not far away at the upper-upstream cylinder (cylinder-3). A free shear layer of the upstream cylinders covers the downstream cylinders at small gap ratio, at intermediate gap ratio it impinges the downstream cylinders, and at a large gap ratio it produces vortices shed near the wake of each cylinder.

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