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 $Re=1500$ , 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 ${\lambda }_{ci}$ are applied to capture vortex structures, and the effects of the gap ratio on vortex evolution behind the cylinder are revealed. (i) For $G/D\le 0.25$ , 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 $0.5<G/D\le 1$ , the lower wake vortex can directly interact with the secondary vortex while the direct interaction is reduced at $1<G/D\le 2$ . (iv) For $G/D>2$ , 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.

The experimental investigation focuses on the effects of a short splitter plate on the flow physics of a circular cylinder in proximity to a wall by particle image velocimetry. The Reynolds number is Re = 3900, and the near-wall cylinder is immersed in turbulent boundary layer flow. Three gap ratios (i.e. $G/D$ = 0.25, 0.5 and 1) are considered, and the splitter plate length is $L/D=0$ , 0.25, 0.5, 0.75 and 1. For $G/D$ = 0.5 and 1, as $L/D$ increases from 0 to 1, the splitter plate facilitates the cylinder shear layers to elongate downstream, and the vortex formation length is increased, which leads to the increase of the range of the recirculation region. For $G/D$ = 0.25, the wall suppression on the wake vortex formation is enhanced, and the variations of the vortex formation length and the range of the recirculation region with $L/D$ are small. The Strouhal number St presents a decrease with increasing $L/D$ for the three gap ratios. The effects of $L/D$ on the vortex evolution are revealed. For $G/D$ = 0.5 and 1, as $L/D$ increases, the induction of the lower wake vortex on the wall secondary vortex becomes weaker due to the reduction in strength of the wake vortex and the increase of the vortex formation length. Additionally, the wake fluctuation intensity is decreased with the increase of $L/D$ due to the splitter plate suppression. For $G/D$ = 0.25, theL/D influences on evolution of the wake vortices and wall secondary vortex are small, which result in weaker variation of the wake fluctuation intensity with $L/D$ .

The flow around two tandem circular cylinders in proximity to a wall is investigated using particle image velocimetry (PIV) for Re = 2 × 10³. The spacing ratios L/D are 1, 2, and 5, and the gap ratios G/D are 0.3, 0.6, and 1. The proper orthogonal decomposition (POD) method and λci vortex identification method are used to investigate the evolution of flow structure, and the influences of L/D and G/D on flow physics are shown. At L/D = 2 and G/D = 0.3, a “pairing” process occurs between the wall shear layer and the upstream cylinder’s lower shear layer, resulting in a small separation bubble behind the upstream cylinder. At L/D = 1, the Strouhal number (St) increases with decreasing G/D. At three gap ratios, the St gradually decreases as L/D increases. At G/D = 0.3, there is nearly a 49.98% decrease from St = 0.3295 at L/D = 1 to St = 0.1648 at L/D = 5, which is larger than the reductions in cases of G/D = 0.6 and G/D = 1. The effects of L/D on the evolution of flow structure at G/D = 0.6 are revealed in detail. At L/D = 1, the vortex shedding resembles that of the single cylinder. As L/D increases to 2, a squarish flow structure is formed between two cylinders, and a small secondary vortex is formed due to induction of the lower shear layer of the upstream cylinder. At L/D = 5, there is a vortex merging process between the upper wake vortices of the upstream and downstream cylinders, and the lower wake vortex of the upstream cylinder directly impinges the downstream cylinder. In addition, the shear layers and wake vortices of the upstream cylinder interact with the wake of the downstream cylinder as L/D increases, resulting in reductions in velocity fluctuations, and the production and turbulent diffusion of turbulent kinetic energy are decreased behind the downstream cylinder.