The gap ratio effects on vortex evolution behind a circular cylinder placed near a wall

Abstract

The vortex evolution behind a circular cylinder placed near a wall is investigated experimentally with particle image velocimetry. The Reynolds number based on the cylinder diameter D is R e = 1500, and the gap ratio G/D varies from 0 to 2.5 (where G is the net gap between the lower surface of the cylinder and the wall). The proper orthogonal decomposition and the vortex identification method λ c i are applied to capture vortex structures, and the effects of the gap ratio on vortex evolution behind the cylinder are revealed. (i) For G/D ≤ 0.25, the regular and alternate wake vortex shedding are suppressed, and the wake vortex shedding is associated with the upper shear layer of the cylinder. The wake vortex is shed from the curling of the upper shear layer and finally breaks down into some smaller vortices at G/D = 0. (ii) For G/D = 0.5, there is a vortex merging process among the upper wake vortex and the secondary vortex; meanwhile, some smaller vortices broken from the secondary vortex are entrained into the recirculation region. (iii) For 0.5 2, the flow of the region is similar to that of an isolated cylinder. The variation of the Strouhal number (St) is dependent on the gap ratio, and the St increases as the gap ratio decreases down to G/D = 0.25, and there is nearly a 92% increase from St = 0.192 at G/D = 2.5 to St = 0.369 at G/D = 0.25, which is caused by the deflection of the gap flow. Moreover, the regular and alternate vortex structure can be formed as the gap ratio increases, and the flow characteristics and the vortex evolution are strongly affected by the gap ratio.