Experimental research on wake characteristics and vortex evolution of side-by-side circular cylinders placed near a wall

Abstract

The wake characteristics and the vortex evolution in flow past the side-by-side circular cylinders placed near the wall are investigated with particle image velocimetry (PIV), with the objective of clarifying the interaction between the side-by-side cylinder wake and the wall boundary layer flow. The Reynolds number based on the cylinder diameter D is 1.5×103, and the gap ratio is G/D=1 where G is the gap size, and three spacing ratios (T/D=3, 1.5, and 1.1) are selected for comparison at δ/D=0.63 where δ is the boundary layer thickness without the cylinder. The vortex identification method λci and the virtual dye visualisation are applied to capture vortex evolution. Firstly, the wake characteristics of the side-by-side cylinder without the effects of wall are investigated, and the cylinder wake can be classified into three typical flow states: double wake for T/D=3, deflected gap flow for T/D=1.5, and micro gap flow for T/D=1.1. Secondly, the wake characteristics and vortex evolution of the side-by-side cylinders placed near the wall are investigated in detail. At T/D=3 and G/D=1, the interaction between the wakes of the two cylinders is weak, and two Karman vortex streets are formed, and the secondary vortex is induced periodically by the wake vortex of lower cylinder. As T/D decreases, the lower wake vortex of upper cylinder directly interacts with the lower cylinder wake vortices, and there is the vortex merging occurred in lower wake vortices of two cylinders, meanwhile, the lifting up of the secondary vortex is delayed for small T/D. In addition, the wake vortex shedding is affected by the wall and the T/D, and the deflection of the gap flow between the two cylinders is the reason for the variation of Strouhal number. As T/D decreases from 3 to 1.5, the magnitudes of Reynolds stresses are decreased, and the distributions of Reynolds stresses are asymmetric about the centreline of cylinder due to the deflection of the gap flow.