Global features of flow around twin cylinders

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 a and T/D investigated are observed, including three kinds of tristable flows and four kinds of bistable flows. The 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. A mutual discussion of forces, St and flow structures is presented in this paper.