Oscillatory flow around a pair of cylinders of different diameters

This paper investigates sinusoidal oscillatory flow around a two-cylinder system numerically. The cylinder configuration comprises a pair of nearby cylinders of different diameters, with the small cylinder piggybacks on the big one. The configuration is often referred to as a "piggyback" cylinder system. The large cylinder is referred as the main cylinder and the piggyback cylinder is referred as the small cylinder. Flow structures around and hydrodynamic loadings acting on the main cylinder arc expected to be influenced by the small cylinder due to the hydrodynamic interactions between the two cylinders. In order to find out the influences induced by the existence of the small cylinder at the proximity of the main cylinder, simulations are carried out both on an isolated cylinder and on the side-by-side piggyback configuration cases. The simulations arc carried out at Reynolds number of 800 and Keulegan-Carpenter number of 8 (both based on the diameter of the main cylinder). In the two--cylinder cases, the small cylinder is located above the main cylinder and the gap to diameter ratio (g/D, where g is the gap and D is the diameter of the main cylinder) is in the range of 0.1 to 0.5 with a uniform increment of 0.1 to investigate the corresponding influences of the gap ratio, The diameter ratio (d/D, where d is the diameter of the piggyback cylinder) is 0.2 in the present study. Hydrodynamic forces and flow characteristics around the cylinders arc investigated in detail. Flow structures near the two cylinders are investigated through flow visualization. It is found that the existence of the small cylinder has profound effects on the flow structures around the main cylinder and hydrodynamic forces on the main cylinder.