The impact of localized surface roughness on the development of a short laminar separation bubble (LSB) forming on the suction side of a NACA 0018 airfoil at an angle of attack of $6^{\circ }$ and a chord-based Reynolds number of ${\text{Re}}_c=100000$ is investigated experimentally. Direct force measurements and two-component particle image velocimetry (PIV) in multiple planes are used to characterize the effect on flow development compared to the baseline flow over a smooth airfoil and fully tripped model. Results show that localized roughness not only eliminates the downstream LSB but also induces significant three-dimensional modifications to bubble topology over a substantial spanwise region ($\sim \mathcal{O}\left(c\right)$) extending beyond the roughness element. This leads to a more substantial impact on aerodynamic performance than that expected from the relative spanwise extent of the localized roughness. The results suggest that bypass transition leads to mean flow reattachment in the spanwise proximity of the localized roughness. The produced three-dimensional, open LSB is characterized by the presence of notable spanwise flow in the separated flow region and spanwise variations of mean separation, transition, and reattachment. Consequently, changes are observed in the formation and development of shear layer vortices along the span, despite the dominant streamwise wavelength and frequency of perturbations remaining nearly unchanged. Furthermore, the modified LSB exhibits relatively low-frequency spanwise “breathing,” which induces mean flow deformations that modulate the stability of the LSB and contribute to the more complex vortex dynamics observed in this flow region.

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• Boundary layer receptivity, stability & separation
• Boundary layers
• Shear flows
• Transition to turbulence
• Vortex dynamics
• Vortex shedding