Numerical study on the aerodynamic characteristics and flow field of two tandem tapered square cylinders with different spacing ratios

Three-dimensional (3-D) large eddy simulations (LES) are carried out to explore the influence of spacing ratio G/ D on the flow structures and aerodynamic interference mechanism of two tandem square cylinders with and without tapered modification (taper ratio xi = 0 % and 5 %) at Reynolds number Re = 2 x 103. Different spacing ratios G/D from 1.4 to 6.0 are considered, where G is the cylinder center-to-center distance and D is the cylinder width. The flow structures, wind pressure distributions, force coefficients, and power spectral density of the two cylinders are comprehensively analyzed and discussed. The mathematical relationship between spacing ratio and force coefficients is established, which can provide theoretical reference for the design and construction of tandem structures. The results indicate that the flow structures and aerodynamic forces are very sensitive to the spacing and taper ratios. The flow structures of the two cylinders can be divided into three flow regimes based on G/D: single body, reattachment, and co-shedding regimes. As the taper ratio increases, the cross-sectional width of the two cylinders gradually narrows along the height, which in turn weakens the interference effect between the two cylinders and decreases the range of the critical spacing ratio. The width of the shear layer becomes gradually narrower and closer to the surfaces of the two cylinders, resulting in a higher frequency and lower energy of vortex shedding. In addition, the mean and fluctuating wind pressure coefficients, the mean force coefficients in the along-wind direction, and the fluctuating coefficients all decrease with the increase in taper ratio, which is attributed to the reduction of the periodicity and consistency of vortex shedding. Meanwhile, there are weaker mean and fluctuating wind pressure and force coefficients for the two cylinders under the reattachment flow regime. Therefore, in the design and construction of tandem structures for ocean engineering, it should be considered to improve the wind resistance performance by making the structures in the reattachment flow regime.