Aerodynamic Characteristics and Wake Evolution of an Asymmetric Rounded Square Cylinder: Roles of Delayed Separation and Shear-Layer Interaction

Corner modification is an effective passive control strategy for mitigating aerodynamic loads on bluff-body structures. The aerodynamic characteristics of an asymmetric rounded square cylinder with rounded upstream corners and sharp downstream corners were investigated numerically at a Reynolds number of 104. Unsteady simulations based on the Transition SST model were performed for corner radius ratios ranging from 0 to 0.20. The results indicate that leading-edge rounding produces a pronounced non-monotonic influence on the aerodynamic characteristics. As the rounding ratio increases, the mean drag coefficient and the root-mean-square lift coefficient are reduced by up to 49.3% and 95.9%, respectively. During this stage, the Strouhal number remains nearly unchanged. Further increases in the rounding ratio result in a recovery of both drag and lift fluctuations, accompanied by a rapid increase in the Strouhal number. Analysis of the wake structure reveals a non-monotonic evolution of the recirculation region, with the recirculation length first increasing and then decreasing sharply as the rounding ratio increases. The wake undergoes a transition in its dominant development mechanism as the recirculation region reaches its maximum extent and the base pressure recovers most effectively. The results suggest that the aerodynamic response is primarily governed by the competition between delayed separation and shear-layer interaction. Delayed separation appears to dominate at moderate rounding ratios, whereas enhanced shear-layer interaction becomes increasingly important at larger rounding ratios. These findings provide physical insight into the aerodynamic optimization of bluff-body structures through asymmetric leading-edge modifications. The proposed wake-transition mechanism is primarily interpreted from the 2D URANS simulations and is supported by representative 3D simulations.