Numerical Investigation of Passive Flow Control over an External Backward-Facing Step Using Rigid and Elastic Plates

This study investigates the passive control of backward-facing step flows using rigid and elastic cantilevered plates, with emphasis on flow reattachment, pressure recovery, and fluid–structure interaction effects. The numerical methodology was first validated against an experimental benchmark, yielding a reattachment-length prediction within 1.28% of the measured value. Following validation, simulations were performed using the unsteady Reynolds-averaged Navier–Stokes equations coupled with the Spalart–Allmaras turbulence model. Rigid plate configurations were examined at momentum-thickness-based Reynolds numbers of Reθ=500, 1000, and 5000, while two-way fluid–structure interaction simulations were conducted at Reθ=5000 to evaluate the influence of structural stiffness. For the baseline configuration, the non-dimensional reattachment length and base drag coefficient remained within the ranges of xr/h=6.18–6.34 and cB=0.199–0.205, respectively. The most effective rigid configuration, L/h=2.5, reduced the reattachment length from xr/h=6.34 to xr/h=5.57 and the base drag coefficient from cB=0.205 to cB=0.175 at Reθ=5000, corresponding to reductions of approximately 12% and 15%, respectively. The fluid–structure interaction simulations showed that plate stiffness strongly influences flow-control effectiveness. The stiffest elastic configuration, with E=2×109 Pa, achieved xr/h=6.14 and cB=0.196, whereas more flexible plates exhibited larger deformation and reduced aerodynamic benefit. Overall, the results demonstrate that cantilevered plates provide an effective passive flow-control strategy for backward-facing step flows. Rigid plates deliver the greatest aerodynamic improvement, while elastic plates require sufficient structural stiffness to maintain favorable pressure recovery and flow-reattachment characteristics.