Infrared thermography measurements of an unsteady laminar separation bubble on a static and pitching airfoil

The present work describes infrared thermography (IRT) measurements conducted on a National Advisory Committee for Aeronautics (NACA) 0018 airfoil model aimed at characterizing a laminar separation bubble (LSB) at low Reynolds numbers under steady and sinusoidal pitching conditions. In steady flow conditions, the accuracy of best practices described in the literature for surface temperature measurements applied to the quantitative assessment of LSB characteristic points (separation, transition, and reattachment) was investigated through a comparative study between IRT measurements and surface flow visualizations. In unsteady flow conditions, the differential infrared thermography (DIT) technique enabled the evaluation of the effects of reduced frequency, oscillation amplitude, and Reynolds number on the behavior of the LSB characteristic point positions along the pitching cycle, with particular emphasis on the reattachment point. The results show a pronounced hysteresis in the characteristic point locations, especially during the upstroke motion, which increases approximately linearly with both pitch rate and oscillation amplitude, while increasing the Reynolds number promotes an upstream shift of the LSB characteristic points. Overall, the present work demonstrates the effectiveness of DIT as a high-resolution, non-intrusive technique for monitoring time-resolved LSB dynamics in unsteady aerodynamic applications.