Fluid dynamics around twin cylinders and interactions

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

Multiple cylindrical structures are widely seen in engineering. Flow interference between the structures leads to a very high fluctuating forces, structural vibrations, acoustic noise, or resonance, which in some cases can trigger failure. Recently circular pins in various arrays have been using as fins to enhance the cooling effect. While the enhancement is directly connected to nature of flow around the pins, no much is known of physics of flow around the pins. The knowledge of flow around two cylinders is insightful for understanding the flow around an array of cylinders/pins. This paper presents results of an experimental investigation into interactions between flowing fluid and a cylinder that is neighbored by another cylinder of the same diameter. Strouhal number (St), time-mean and fluctuating forces on and flow structures around the cylinder are investigated while the gap-spacing ratio T/D is varied from 0.1 to 5 and the attack angle a from 0 to 180 where T is the gap width between the cylinders, and D is the diameter of a cylinder. A flow visualisation test was conducted to observe flow structures around the cylinders. Based on forces, St, flow structures and fluid-cylinder interaction mechanisms, 19 distinct flow categories in the ranges of a and T/D are observed, including one quadristable flow, three tristable flows and four 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/drop in forces and St of the cylinders. Six different interaction mechanisms are observed, namely interaction between boundary layer and cylinder, shear layer/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/wake and cylinder suppresses mean and fluctuating forces as well as weakens flow unsteadiness for stationary cylinders but may cause violent galloping vibration when the cylinders are elastic. The interaction between boundary layer and cylinder also may generate galloping vibrations.