LAUSR

Abstract Experiments were conducted to study the flow around an array of four square cylinders arranged in square configuration under a subcritical Reynolds number Re = 8000. The goal is to systematically examine the effects of the cylinders’ center-to-center pitch (with 6 pitch ratios P / D  = 2–5) and the array orientation with reference to oncoming flow (with 7 incidence angles α = 0°–45°), i.e., 42 cases in total. The flow field was measured using digital Particle Image Velocimetry (PIV), in conjunction with direct measurement of the hydrodynamic forces (lift and drag) on each cylinder using a piezoelectric load cell. The flow interference among the cylinders in the array is complex, which could be non-synchronous, quasi-periodic or synchronized. The flow patterns, Reynolds shear stress distributions, temporal variation and power spectra of drag and lift coefficients on each cylinder, force statistics and vortex shedding frequencies are discussed in detail. Similar to that of four-circular-cylinder array, the flow could be broadly classified into three regimes (shielding, reattachment and impinging) governed by the combined value of P / D and α. However, it differs noticeably from that of the four-circular-cylinder array, including: (i) a lack of reattachment regime for the inline square configuration (α = 0°); (ii) the unique feature of attractive lift for the pair of cylinders arranged side-by-side; and (iii) higher sensitivity to the variation in incidence angle. A new criterion is proposed to quantitatively classify the different flow regimes based on the averaged RMS drag/lift coefficients of the four cylinders in the array.