Aspect ratio and interference effects on flow over two rectangular cylinders in tandem arrangement

Flow past twin tandem rectangular cylinders for various gap ratios (L* = 1.0-8.0) and aspect ratios (B* = 0.3-4.0) at a Reynolds number of 150 was investigated via 2D direct numerical simulation. Results showed that based on the variation trend of fluid force with B*, three types of gaps spacing are categorized, i.e., narrow gap (L* =1.0, 2.0), medium gap (L* = 3.0, 4.0), and wide gap (L* = 6.0, 8.0). Higher B* reduces the fluctuating force of downstream cylinder (DC) for narrow gaps, but amplifies that of the DC for wide gaps. For medium gaps, variation of fluid forces with B* < 1 is in line with that of wide gaps, while it resembles that of narrow gap type at B* > 1. Moreover, gap flow is classified into three flow regimes, i.e., extended body, reattachment, and co -shedding. Wake flow structures of DC show 2S vortex type at narrow gaps for all B* cases, whereas they transfer from synchronized C(2S) to 2S type when B* > 0.3 for medium and wide gaps. Proper orthogonal decomposition analysis captures the spatial distribution and temporal evolution of coherent structure hidden in flow field; thus, it well explains the fundamental mechanism that fluid force, gap flow and wake vortex change with B* and L*.