Flow-pattern identification for two staggered circular cylinders in cross-flow

The how around two circular cylinders of equal diameter, arranged in a stag gered configuration, was investigated using flow visualization and particle image velocimetry for centre-to-centre pitch ratio P/D = 1.0 to 5.0 and an gle of incidence alpha = 0 degrees to 90 degrees. Experiments were conducte d within the low subcritical Reynolds number regime, from Re = 850 to 1900. Nine flow patterns were identified, and processes of shear layer reattachm ent, induced separation, vortex pairing and synchronization, and vortex imp ingement, were observed. New insight was gained into previously published S trouhal number data, by considering the flow patterns involved. The study r evealed that vortex shedding frequencies are more properly associated with individual shear layers than with individual cylinders; more specifically, the two shear layers from the downstream cylinder often shed vortices at di fferent frequencies.