Wake-induced instabilities of parallel circular cylinders with tandem and staggered arrangements

When two circular cylinders are closely arranged in parallel, the downstream cylinder frequently exhibits wake-induced vibrations (WIVs). This study investigates the classifications of WIVs in various arrangements of cylinders by considering the generation mechanisms of WIVs through a series of wind tunnel tests. The generation mechanisms are defined through flutter and time-history analyses using the quasi-steady theory. Finally, this study suggests a classification map of the arrangements with four types of generation mechanisms: vertical flutter, anti-phase coupled flutter, in-phase coupled flutter (quasi-steady applicable), and in-phase coupled flutter (quasi-steady inapplicable). Vertical flutter occurs in closely staggered cylinders and tandem cylinders with a horizontal center distance measuring 2.5–9.5 times the cylinder diameter. A key parameter for distinguishing coupled flutter is the contribution of the aerodynamic coupling terms. Furthermore, for closely staggered cylinders, two types of vibrations occur, depending on the reduced wind velocity. The arrangement of the two cylinders is slightly changed because of the static displacement of the downstream cylinder depending on the wind velocity, resulting in a change in the generation mechanism. The results pertaining to the generation mechanisms effectively explain the WIV responses of the two cylinders in various arrangements