Influence of spacing ratio on vortex shedding regimes in cylinder/prism tandem configurations

The complex interference effects in flows around multiple bluff bodies significantly influence structural safety. While previous studies have investigated aerodynamic loads of the downstream body, the intrinsic relationship between these responses and flow evolution remains insufficiently understood. This study systematically investigates the flow around circular and square cross section bluff bodies in various tandem arrangements at Re = 1.2 × 104 through wind tunnel experiments. It specifically elucidates the causal relationship between vortex shedding evolution and the critical spacing ratio for abrupt changes in the downstream body's aerodynamic forces, as well as the dependence of this critical transition on the upstream cross-sectional geometry. The results reveal that, across all tandem arrangements, both the surface pressure distribution and aerodynamic forces on the downstream body undergo sharp transitions when the spacing ratio crosses a distinct critical value. This abrupt change is consistently accompanied by a significant reorganization of the vortex shedding pattern. Furthermore, when the downstream body is a square prism, the critical spacing ratio remains stable within the range of 3.5 < P/D ≤ 4, regardless of whether the upstream body is a circular or square cylinder. Flow field analysis indicates that this phenomenon occurs because the wakes generated by different upstream bodies evolve into similar inflow conditions at the location of the downstream square cylinder, which combines with the fixed flow separation points inherent to the square prism. Consequently, the square prism, with its fixed flow separation points, exhibits consistent aerodynamic transition behavior under these similar inflow conditions.