Transitions in the unsteady wakes and aerodynamic characteristics of the flow past three square cylinders aligned inline

A numerical study is performed to analyze the effect of low Reynolds numbers (Re) and gap spacings (g  ) on the flow around three square cylinders aligned inline using the multi-relaxation-time lattice Boltzmann method (MRT LBM). The Reynolds number is varied from Re=90Re=90 to 175 while the spacing between the cylinders is taken in the range 0.5≤g≤60.5≤g≤6. It is found that MRT LBM captures the flow characteristics efficiently. Seven different flow patterns are revealed in this study at different Re and g   values. Variation of different aerodynamic force coefficients, like mean drag coefficient, Strouhal number, root mean square values of drag and lift coefficients, with flow patterns are discussed in detail in order to develop a link between flow patterns and force characteristics. The drag inversion (DI) spacing (or critical spacing) for the middle cylinder lies within the spacing range 2≤g≤32≤g≤3 and depends on Re. The effect of Re on DI is also studied and it is concluded that the DI spacing decreases with increment in Re. At all chosen Re   it is found that the Strouhal number decreases up to DI spacing value while it has increasing behavior after that. The negative values of average drag coefficient for third cylinder are also observed either at low spacing or at high spacing values. Re=90Re=90 and 140 are found to be critical for different wake characteristics. Generally, it is observed that the upstream cylinder has higher drag force compared to the middle and downstream one but at some Re and g values the middle or downstream cylinder also found to have higher drag force than the upstream one. The root mean square value of lift coefficients for all the cylinders is higher than the corresponding root mean square value of drag coefficients.