Rectangular cross-section cylinders with high slenderness ratios are commonly employed in wind-sensitive structures such as super-tall buildings and long-span bridges. Two-dimensional laminar and three-dimensional large eddy simulation (LES) were carried out to investigate the Reynolds number(Re)effects of rectangular cylinders with an aspect ratio B/D=1/3~6 and Re=100~120 000. The mean drag coefficient (CD), Strouhal number (St), mean pressure coefficient (Cp), and flow mechanism of rectangular cylinders were analyzed. It is found that within the ranges of B/D and Re examined in this study, the flow characteristics around rectangular cylinders can be classified into three categories according to B/D. For B/D<2.5, there is no reattachment and flow transition from laminar to turbulence occurring in the range of Re=100~250. As Re increases, the negative pressure on the back surface of the rectangular cylinder rises, leading to an increase in CD. For 2.5<B/D≤6, separated shear layers reattach to the lateral surfaces, and flow transition occurs from Re=500 to 1 000, accompanied by significant Reynolds number effects on CD and St. However, these effects become negligible when Re≥1 000. Notably, for B/D=2.5, shear layer reattachment occurs at Re=250~500, the streamwise length of wake flow decreases with increasing Re; when Re≥1 000, the reattachment phenomenon disappears, and the streamwise length of wake flow significantly increases.
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