Infrared Thermography Imaging of an Unsteady Laminar Separation Bubble on a NACA 0018 Pitching Airfoil

This study investigates the dynamics of a Laminar Separation Bubble (LSB) on a NACA 0018 airfoil at low Reynolds numbers, comparing steady-state conditions with sinusoidal pitching motions to enhance aerodynamic performance characterization. By employing Infrared Thermography (IRT) and its unsteady extension, Differential Infrared Thermography (DIT), the research identifies the critical separation, transition, and reattachment points that define the LSB structure. Wind tunnel tests were conducted at Politecnico di Milano utilizing a 3D-printed model and a high-sensitivity FLIR A6781 SLS camera to capture time-resolved thermal footprints under varying reduced frequencies, oscillation amplitudes, and Reynolds numbers. The analysis, processed through custom mapping and dewarping algorithms, reveals a significant hysteresis in the bubble's position during the pitching cycle, where the lag between upstroke and downstroke phases increases linearly with pitch rate and amplitude. While steady-state IRT remains a benchmark for validation, this work highlights that DIT is a robust, non-intrusive diagnostic tool capable of tracking the transient behavior of transition and reattachment points, providing an essential framework for validating numerical tools and understanding complex boundary layer phenomena in unsteady aerodynamic applications.