Flow past two wall-mounted square cylinders in a tandem arrangement are simulated through direct numerical simulation to investigate the effects of the gap between the two cylinders on the wake. Numerical simulations are conducted for a constant Reynolds number of 500, height to width length ratio H = 4, and gap to width ratios of G = 1 to 8 with an interval of 1. The flow in the wake of the downstream cylinder is found to be significantly affected by the free shear layers from the top and sides of the upstream cylinder. At G = 1 and 2, the free-shear layer generated from the upstream cylinder reattaches the top surface of the downstream cylinder and further develops into a downwash behind the downstream cylinder. At G = 3 to 8, the downwash behind the downstream cylinder disappears because flow separation from the top upstream edge of the downstream cylinder does not occur for G = 3 to 6 and is very weak for G = 7 and 8. The disappearance of downwash in the wake of the downstream cylinder further results in very weak variation of flow along the span of the downstream cylinder. The single, reattachment, and binary wake modes at the mid-span of the cylinder occur at G = (1 and 2), (3 and 4), and (5 and above), respectively.

Flow past two wall-mounted square cylinders in staggered arrangement is simulated through direct numerical simulation to investigate the effects of the degree of staggering on the wake and forces compared to the tandem arrangement. Numerical simulations are conducted for a constant Reynolds number of 500, height to width ratio H = 4, staggered distance to diameter ratios S = 0.5 and 1, and gap to width ratios G in the range from 1 to 8. It is found that moving the downstream cylinder sideway for S = 0.5 and 1 changes flow features and force coefficient significantly. At small gaps of G = 1 to 3, the shear layer from one side of the upstream cylinder is trapped into the gap. In addition, a strong horseshoe vortex in front of the downstream cylinder is found for all the gaps at S = 0.5 and 1, but not found at S = 0. The friction velocity on the plane boundary caused by the horseshoe vortex of the downstream cylinder is greater than that of a single cylinder at S = 1 and G = 1 to 8 and at S = 0.5 and G = 2 to 5. At G = 1 to 3, vortex shedding is caused by the trapped flow into the gap on one side of the downstream cylinder and by the approaching flow on another side. The negative mean drag coefficient at small gaps was found in the tandem arrangement case, but not found at S = 0.5 and 1.

No abstract

No abstract