Determination of flow configurations and fluid forces acting on two staggered circular cylinders of equal diameter in cross-flow

Experiments have been carried out to investigate characteristics of flow and fluid forces acting on two circular cylinders in a wide range of staggered configuration. Time-averaged and fluctuating fluid forces acting on the upstream and downstream cylinders were measured for staggered angles α=10°, 25°, 45°, 60° and 75° in the range of T/D=0.1-5.0, where α is the angle between the free-stream flow and the line connecting the centers of the cylinders, T is the gap width between the cylinders, and D is the diameter of a cylinder. Fluid forces acting on the cylinders in staggered configurations were measured in wind-tunnel tests at a Reynolds number of 5.5×104 and, then the dependence of magnitudes and trends of fluid force coefficients on the spacing ratio T/D are discussed in light of flow configurations determined on the basis of time-averaged and fluctuating surface pressure, surface oil-flow patterns, and flow visualization patterns. At very small T/D, lift forces depend largely on the gap flow between the cylinders, irrespective of the α value. It is found that the intermittent formation and burst of the separation bubble on the upstream or downstream cylinder is responsible for the bistability of the flow. The flow structure jump from one stable state to another appears discontinuous and is always associated with a change from the presence to absence of a separation bubble around a cylinder or vice versa. Maximum fluctuating drag force acting on the downstream cylinder occurs at α=10°, T/D=2.4-3.0 where the inner shear layer of the upstream cylinder rolls just a little forward of the front surface of the downstream cylinder.