串列梯形柱绕流水动力特性数值分析

Flow over two tandem trapezoidal cylinders with gap ratio of 3 at Reynolds number of 150 is numerically investigated. When the length ratio of the rear base to the front one is d/D = 0, 0.3 or 0.5, the merging of vortices shed from the upper surface of the two cylinders results in the asymmetric P+S shedding mode in the wake. Consequently, the mean lift imposed on the downstream cylinder deviates from the zero. The wake evolves into 2S mode when d/D=0.7, presenting the alternate shedding after vortices merging from both sides. At d/D=1, the quasi-stationary vortices are observed between the two cylinders and in the wake of the downstream cylinder. Additionally, no vortices are formed in the wake although there is a slight flapping for the shear layers. At d/D=0 and d/D=0.3, the boundary layers reattach the lateral edges of the upstream cylinder, generating the subordinate vortices and hence an increase in local pressure. With increasing d/D, the vortex formation length of the upstream cylinder elongates, and the gap flow becomes steadier with the weakening of the transverse velocity fluctuation in front of the downstream cylinder. As a result, the wake flow becomes more symmetrical. The time-averaged drag of the upstream cylinder is larger than that of downstream one due to the shielding effect. The flow around two tandem square cylinders presents a quasi-stable state, resulting in the much smaller hydrodynamic coefficients as compared to those of triangular and trapezoidal cylinders. In comparison with tandem circular cylinders, the present trapezoidal and triangular cylinders experience the more forward separation of boundary layer, resulting in larger drag and lift forces.