On the mitigation of detrimental effects via passive flow-control devices in aerodynamic airfoils operating under turbulence and surface degradation effects

This work investigates how moderate turbulence and surface roughness jointly affect the aerodynamics of a NACA0021 airfoil operating at transitional Reynolds numbers (Re=0.8–1.2×105). Wind-tunnel tests were performed under two controlled flow paradigms: a low-disturbance Clean condition and a Combined condition including both freestream turbulence (I≈3.8%) and leading-edge roughness. In addition to the baseline (Bare) configuration, two unilateral passive device (PD) layouts were tested, with triangular protrusions placed at x/c=0.25 (PD25) and x/c=0.5 (PD50). Results show that the Combined inflow linearizes the lift curve and softens stall but reduces lift slope and aerodynamic efficiency by up to 60% compared with the Clean baseline. The effect of the PDs depends on their position and on Re: PD25 can delay stall and recover efficiency at high angles of attack, while PD50 offers only limited or negative impact once the boundary layer becomes fully turbulent. Overall, the findings demonstrate that turbulence–roughness interactions dominate aerodynamic behavior at transitional scales, and that small passive elements can provide only local improvements. These results underline the need to assess low-Re airfoils under realistic inflow conditions when developing flow-control strategies for distributed or urban wind-energy systems.