Flow configuration improvement and vortex evolution of staggered cylinders

Wind tunnel experiments were performed to collect surface pressure data on twin staggered cylinders with longitudinal pitch ratios (L/D) of 2.4–7.7 and transverse pitch ratios (T/D) of −4 to 0 at Re = 4 × 10⁴. Simultaneous measurements of the pressure and velocity fields were performed for typical flows. Seven distinct flow configurations were identified and classified, including a newly discovered one termed the shear layer low-frequency swinging (SLS) flow configuration. This configuration arises from the periodic swinging of the inner shear layer of the downstream cylinder, alternating between vertical and horizontal orientations with respect to the flow direction at a Strouhal number of 0.03. The surface pressure properties of the downstream cylinder are analyzed by investigating the vortex formation and evolution. Various vortex evolution processes have been discussed, including periodic and persistent reattachment, incident vortex collisions, gap-vortex pairing and rushing, high-frequency vortex merging, and high-frequency modulation. The high-frequency modulation interrupts the shedding of low-frequency vortices from the outer side of the downstream cylinder, leading to the intermittent formation of two vortices during one low-frequency period. Except for the twin street flow configuration, each shear layer of the downstream cylinder sheds vortices at different rates in the two-frequency case.