Characteristics of the flow around two tandem cylinders of triangular and circular shapes

This study investigates the influence of the triangular cylinder's shape on drag reduction, frequency suppression, and flow regimes of a downstream circular cylinder in tandem arrangements. A non-iterative boundary thickening-based immersed boundary method coupled with the characteristic-based split finite element method is developed to effectively enforce boundary conditions and maintain the flow stability. Its accuracy and robustness have been validated against benchmark problems. The effects of the triangular cylinder's aspect ratio (HTri), shape factor (Wcur), and spacing between the cylinders (DCT) are systematically analyzed. The results show that increasing the aspect ratio or adjusting the shape factor of the triangular cylinder can effectively reduce the lift and drag coefficients acting on the downstream cylinder. The Strouhal number (St) initially increases as HTri increases and then stabilizes at a higher value. At small spacing (DCT≤2D), the shear layer reattachment regime (regime I) transitions from alternating vortex reattachment to simultaneous vortex reattachment as HTri increases. At larger spacing (DCT=3D), the flow transitions to a co-shedding regime (regime II), characterized by a transition from primary vortex shedding mode to the two-layer vortex shedding mode with increasing HTri. A critical spacing range (2D<DCTc≤3D) for wake pattern transitions is identified, highlighting that flow regime control can be achieved solely through triangular cylinder shape modulation without adjusting spacing. These insights are valuable for the design and optimization of tandem configurations aimed at achieving effective drag reduction, frequency control, and improved flow management in multi-object flow systems.