Abstract
Cylindrical structures in a group are frequently seen on land and in the ocean. Mutual flow interaction between the structures makes the wake very excited or tranquil depending on the spacing between the structures. The excited wake-enhancing forces in some cases cause a catastrophic failure of the structures. This paper presents results of an experimental investigation of Strouhal number (St), time-mean, and fluctuating forces on, and flow structures around, two identical circular cylinders at stagger angle α = 0°-180° and gap-spacing ratio T/D=0.1-5, where T is the gap width between the cylinders, and D is the diameter of a cylinder. While forces were measured using a load cell, St was from spectral analysis of fluctuating pressures measured on the side surfaces of the cylinders. A flow visualization test was conducted to observe flow structures around the cylinders. Based on forces, St, and flow structures, 19 distinct flow categories in the ranges of α and T/D investigated are observed, including one quadristable flow, three kinds of tristable flows, and four kinds of bistable flows. The quadristable, tristable, and bistable flows ensue from instabilities of the gap flow, shear layers, vortices, separation bubbles, and wakes, engendering a strong jump or drop in forces and St of the cylinders. The two cylinders interact with each other in six different mechanisms, namely interaction between boundary layer and cylinder, shear layer or wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex. While the interaction between vortex and cylinder results in a very high fluctuating drag, that between vortex and shear layer results in a high fluctuating lift. On the other hand, the interaction between shear layer or wake and cylinder weakens mean and fluctuating forces and flow unsteadiness. A mutual discussion of forces, St, and flow structures is presented in this paper.
Highlights
Slender structures in groups have many engineering applications, for example, chimney stacks, tube bundles in heat exchangers, high-rise buildings, harvesting wave and tide energy from ocean, overhead power-line bundles, bridge piers, stays, masts, chemical-reaction towers and offshore platforms
Fluid forces, Strouhal numbers (St) and detailed insight into flow structures and their instability are investigated for two circular cylinders of identical diameters for α = 0, 10, 25, 45, 60, a 90, 105, 75, 120, 135, 155, 170 and 180 and T/D = 0.1 ~ 5.0
Quadristable flow: it is afoot in side-by-side arrangement at small T/D ( = 0.1 ~ 0.2), resulting from simultaneous instabilities of the gap flow and separation bubble
Summary
Slender structures in groups have many engineering applications, for example, chimney stacks, tube bundles in heat exchangers, high-rise buildings, harvesting wave and tide energy from ocean, overhead power-line bundles, bridge piers, stays, masts, chemical-reaction towers and offshore platforms. Recent experiment by Alam et al [20], involving mean and fluctuating force measurements, pressure measurements, surface oil-flow visualization and conventional flow visualization, have provided more physical insight into the origin of the ‘inner- and outer-lift peaks’ They presented and discussed their results with variation in T/D, failing to explicate global features of flow with variations of both T/D and α and to classify flow regime globally. Flow visualization and surface oil-flow techniques are employed to get insight into the physical flow around the cylinders and to get a better understanding of the mean and fluctuating forces and St. The objectives of this study were (i) to classify the flow regime globally based on forces, St and flow structures (ii) to elucidate the flow structure for each regime, (iii) to find possible interaction mechanisms, and (iv) to make a correlative discussion of flow, forces and St, for the whole range of α = 0 ~ 180 and T/D = 0.1 ~ 5.0. The linkage between force and flow structure and possible interactions is discussed in detail
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