Spanwise resolution effects on flow simulations around a square cylinder at the critical angle

In large-eddy simulations or direct numerical simulations of flows past bluff bodies, the spanwise resolution significantly increases the total cell count and, consequently, the computational cost. The spanwise resolution dz is normalized by the characteristic length scale D (width or diameter), yielding the nondimensional form dz/D. This study investigates the influence of dz/D on the separating–reattaching flows around an infinitely long square cylinder at the critical incidence angle of 14° and at a high Reynolds number of 2.2×104. Simulations span dz/D=0.013–0.08 to systematically evaluate resolution effects. The results demonstrate significant effects of dz/D on aerodynamic forces, surface pressures, and the flow statistics, even at spanwise resolutions finer than those employed in most previous studies. Coarser resolutions (dz/D>0.013) yield higher mean drag, larger rms pressure and force fluctuations, and a shorter, narrower recirculation zone behind the cylinder. As dz/D is refined, the shear layer evolves to exhibit both spanwise and streamwise vortices, enhancing its spanwise coherence and resilience to external disturbances. A detailed comparison with fully separated flows reveals both similarities and distinctions in spanwise resolution effects. Key differences include the trailing-edge effects, a markedly higher frequency of the Kelvin–Helmholtz instability, and pronounced low-frequency unsteadiness inherent to separating–reattaching flows.