Effects of sinusoidal gust on the flow structure and reattachment characteristics of two tandem 5:1 rectangular cylinders

This study employs three-dimensional large eddy simulation to investigate the effect of sinusoidal-gust amplitude (AR) on the flow structure around two tandem 5:1 rectangular cylinders. The simulations are conducted at a Reynolds number of Re = 20 000, with gap widths of G = 2D, 6D, and 10D (where D is the cylinder height) and gust amplitudes AR ranging from 0.05 to 0.30. The results reveal that the flow fields are sensitive to AR. An increase in AR promotes the flow pattern transition from either reattachment (at G = 2D) or coshedding pattern (at G = 6D and 10D) to a more unstable state, accompanied by a breakdown of stable two-dimensional vorticity structure into significant spanwise deformation. The surface flow exhibits drastic changes in the recirculation bubbles. For the upstream cylinder, the bubble shrinks gradually with AR, irrespective of the overall flow pattern. In contrast, the recirculation on the downstream cylinder surface presents three distinct variations with AR at the three widths. Quantitative analysis of surface flow further quantifies the dependence of LR on AR. The linear decay slope of LR with AR is approximately −4.95 at all gap widths for the upstream cylinder; while the LR of the downstream cylinder varies with G as follows: increasing linearly from 0 under G = 2D, stabilizes at around 1.45D under G = 6D, and first decreases and then increases under G = 10D. Consistent with these flow-structure changes, the mean drag and fluctuating lift also show marked gap-dependent variations. This comprehensive exploration provides fundamental insights into how sinusoidal unsteady effects modulate the flow structure around tandem rectangular cylinders.