Aerodynamic characteristics of two side-by-side circular cylinders and application of wavelet analysis on the switching phenomenon

The aerodynamic characteristics of two stationary cylinders, both circular and square, in a side-by-side arrangement were investigated experimentally in a uniform flow at a Reynolds number of 5.5 x 10(4), although the square cylinder results are rather limited. This Reynolds number is within the range in which fluid forces acting on a single cylinder are comparatively insensitive to change in the Reynolds number. The focus of this study was on the determination of the characteristics of steady and fluctuating fluid forces, wake frequencies and switching phenomena in two side-by-side cylinders. For a spacing ratio of T/D < 1.2 (T, gap spacing between cylinders; D, diameter), the gap flow was biased to one side, resulting in the formation of a narrower wake behind one cylinder and a wider wake behind the other. Steady and fluctuating fluid forces acting on the cylinders were decomposed for the narrower wake and the wider wake flow patterns. For T/D > 0.20, the action of lift forces on both cylinders was in an outward direction (repulsive); however, for T/D = 0.10, the action of lift force on the cylinder associated with the narrower wake was inward and that on the other cylinder was outward. In the bistable flow regime, T/D = 0.2-1.2, when the gap flow switched spontaneously from one side to the other, another short duration stable flow pattern (which can be termed an intermediate flow pattern) persisted in the intermediate time, in which the gap flow was oriented parallel to the free-stream flow and the Strouhal number was almost equal to that of a single cylinder. Flow visualization images and results of modal analysis and wavelet analysis of velocity signals obtained from two hot-wires corroborated the appearance of the intermediate flow pattern. In the case of synchronized vortex shedding and even in the intermediate flow pattern, a predominant antiphase synchronized vortex was found in both the case of two side-by-side circular cylinders and the case of two side-by-side square cylinders. (C) 2003 Elsevier Ltd. All rights reserved.