Experimental study of vortex interactions in medium-sized circular cylinder arrays under subcritical flow conditions

This study presents an experimental investigation of the flow behaviour in a medium-sized in-line array of circular cylinders (2 × 5) at subcritical Reynolds numbers ( R e = 3900 and R e = 10000 ). Time-resolved Particle Image Velocimetry (TR-PIV) was employed to capture unsteady velocity fields and vortex dynamics. The analysis focuses on the influence of the longitudinal and transverse pitch ratio ( G / D and L / D ) on the wake interaction and turbulence development. The results show that smaller spacing promotes strong wake interference, vortex merging, and elevated turbulent kinetic energy in the gap region. In contrast, larger spacing leads to more distinct wakes, reduced turbulence levels, and faster velocity recovery downstream. Increasing Reynolds number intensifies shear-layer instability and turbulence, while the overall wake topology remains governed by geometric spacing. The gap region is identified as the primary zone controlling instability and mixing, driven by strong velocity gradients and shear-layer interactions. These findings demonstrate that geometric configuration is the dominant parameter governing wake-interference regimes in medium-sized arrays. By bridging the gap between small-scale cylinder interactions and the more homogeneous flows observed in large arrays, this work contributes valuable insight into transitional wake dynamics in intermediate-sized configurations.