Vortex evolution of flow past the near-wall circular cylinder immersed in a flat-plate turbulent boundary layer

Abstract

The vortex evolutions and the turbulent characteristics in flow past the near-wall circular cylinder immersed in a flat-plate turbulent boundary layer are investigated with particle image velocimetry (PIV), with the objective of clarifying the interaction between the turbulent boundary layer flow and the near-wall cylinder wake. The Reynolds number based on the cylinder diameter D is 1.0×103, and three gap ratios (i.e., G/D=0.5, 1 and 2) are chosen for comparison at δ/D=6.7 where G is the gap size and δ is turbulent boundary layer thickness without the presence of cylinder. The proper orthogonal decomposition (POD) analysis is used to obtain phase-averaged flow fields, combining with the vortex identification method λci, which can capture effectively the evolution process of vortex. For three gap ratios, the cylinder is placed at the logarithmic-law layer of incoming flow, and the Karman-vortex sheets can be generated from the curling of the cylinder shear layers. At G/D=0.5 and G/D=1, the secondary vortex can be formed periodically in the wall due to the induction of the lower wake vortex. The vortex evolutions and turbulent characteristics are significantly affected by incoming turbulent boundary layer flow. The swirling strength of the wake vortices shed from the cylinder is enhanced with increasing G/D, and the swirling strength of the upper wake vortex is slightly bigger than that of the lower wake vortex due to velocity differences of incoming flow. The variation of the Strouhal number (St) is dependent on the gap ratio, and there is nearly 10.4% increase from St=0.211 at G/D=2 to St=0.233 at G/D=0.5, which is caused by the combined effects of the deflected gap flow and the velocity gradient. At large gap ratios, especially at G/D=2, the upstream high-velocity fluids interact with the cylinder wake, leading to strong velocity fluctuations behind the cylinder, and the magnitudes of production and turbulent diffusion in turbulent kinetic energy are increased, which also enhances turbulent transports behind the cylinder.