Flow Interference Between Two Tandem Square Cylinders: Passive Control Using a Downstream Flat Plate

Flow interference among bluff bodies can strongly amplify or suppress unsteady aerodynamic forces and induced vibrations thereof. However, the behaviour of tandem square-cylinder interference and its passive control in the laminar regime remains insufficiently quantified, particularly near the known critical tandem spacing ratio (L/D ≈ 4.5). This study systematically analysed and determined how a splitter plate placed downstream of the second cylinder modulates the aerodynamic forces, symmetry of vortex shedding, wake topology, and associated wake metrics in two-dimensional incompressible laminar flows. Unsteady finite-volume numerical simulations were conducted using ANSYS Fluent (2022 R1) at Re = 150. The tandem cylinder spacing (L) was varied over L/D = 2–6 (D, the cylinder side length), and the splitter gap (G) was varied over G/D = 1–6. The splitter plate acted as a strong wake stabiliser at small gaps (G/D = 1), where vortex shedding was largely suppressed and lift fluctuations were minimal (for example, Cl,rms ≈ 0.055), and the DC experienced negative drag (Cd ≈ −0.13), consistent with elongated and weakly rolled-up shear layers and extended recirculation. The splitter plate acted as a sharp control “switch” at a critical splitter gap G/D ≈ 2, where the wake transitioned to unsteady shedding, the Strouhal number (St) jumped to values that remained nearly constant for G/D = 2–6, and the wake metrics indicated earlier roll-up (reduced vortex formation length and recirculation length) and greater lateral spreading (increased wake width). The outcome was influenced by the tandem regimes: for 1.5 < L/D < 4, persistent shielding and negative downstream drag predominated. However, near the critical gap (L/D ≈ 4.5), the restoration of shear-layer impingement at G/D ≥ 2 resulted in a downstream drag surpassing the isolated-cylinder baseline (Cd,SC ≈ 1.49) by approximately 3–8%. In the co-shedding regime, when L/D = 5, there was a notable increase in drag, approximately 12.3% more than that on an isolated cylinder. Conversely, when L/D = 6, the system approached aerodynamic independence without any amplification (approaching the single-cylinder value). The interference metrics showed a maximum combined drag reduction of ~68.6% at L/D = 4.5 and G/D = 1, whereas the upstream cylinder drag was only weakly affected. The results of the present study establish splitter placement as an effective passive control method for suppressing or recovering interference-driven unsteadiness, thereby supporting designs in bluff-body aerodynamics, heat-transfer equipment, and vibration-mitigation systems.