Lattice Boltzmann method based analysis of flow regime transitions around inline rectangular cylinders

This study numerically investigates incompressible two-dimensional laminar flow around two rectangular cylinders arranged in tandem configuration using the lattice Boltzmann method ( LBM ). Simulations are performed at a constant Reynolds number Re = 200 while the gap ratio ( G ) between the cylinders is varied in the range 0 ≤ G ≤ 10. The aspect ratio ( AR ) of both cylinders is fixed at 3:2. Effects of cylinder spacing on wake development, vortex formation, pressure distribution, and streamline topology were systematically analyzed. The prominent flow regimes revealed from computations include the single slender body, alternate reattachment flow, intermittent shedding and the binary vortex street. The power spectrum of the lift coefficient ( CL ) reveals frequency variations in lift for both cylinders across the different regimes. An increase in Strouhal number ( St ) correlates with gap spacing, linked to pronounced bending of shear layers detaching from the cylinders. Pressure distribution is found to be non-uniform due to irregular shear layer separation and reattachment. Critical gap spacing values of G = 2, 3, and 9 are identified as pivotal for abrupt changes in wake modes and pressure distributions, where fluid force parameters exhibit local maxima, minima, or sudden extreme jumps. The second cylinder encounters a net reverse drag force within the range of G = 0 to 3, whereas the first cylinder exhibits positive average drag force across all values of G . Findings indicate that these rectangular shapes experience significant alterations in flow structures and force fluctuations compared to their square counter parts.