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.