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Small-scale steps are common surface imperfections on aircraft wings arising during manufacturing and service, yet how they affect shear-layer development, promote local separation, and alter aerodynamic performance under pressure gradients remains insufficiently understood. This study investigates the aerodynamic performance degradation mechanism of a low-speed airfoil with small-scale backward-facing steps located at 8% c on the suction side using the improved delayed detached eddy simulation method for different step heights. The influence of step height on the behaviors of local separation bubbles and boundary-layer development is systematically analyzed. The results show that the presence of a step generally leads to lift reduction and drag increase, with the severity of these effects governed by the scale relationship between the step height and the local boundary-layer thickness. When the step height exceeds the local boundary-layer thickness, the lift-to-drag ratio decreases by up to 8.17%, accompanied by the delay of reattachment and the generation of an enlarged separation bubble. The behavior is driven by the reorganization of the near-wall shear layer and the amplification of vortex disturbances under this scale relationship, which promotes earlier thickening of downstream boundary-layer, then increases near-wall skin friction, thereby causing a more pronounced degradation of aerodynamic performance.
Chen et al. (2026) studied this question.