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Description
High-lift capability remains a key requirement for aircraft during take-off and landing, traditionally achieved through passive high-lift devices such as slats and flaps that, over the years, have undergone continuous refinement, leading to increasingly efficient but also mechanically complex systems [1]. Active flow-control concepts, such as circulation control (CC), offer a lighter and mechanically simpler alternative. This work presents an Infrared Thermography (IRT)-based experimental investigation of boundary-layer transition and separation on a high-lift airfoil whose shape and active flow control were contemporarily optimized, particularly a tangential jet is issued near the trailing edge, on the airfoil upper surface, to enhance circulation, sustain attachment, and delay separation. The tests were carried out in two configurations: Jet-On and Jet-Off. To evaluate the effect of tripping adding disturbances, a boundary-layer trip was applied at 10% of the chord and extended over 10% of the chord length. Two regions are of interest, one affected by a natural transition and the other by a forced one. The IRT tests were conducted in parallel with Wall-Pressure Measurements (WPM) and Particle Image Velocimetry (PIV) campaigns.
IRT provides non-intrusive, full-field visualization of boundary-layer state by exploiting the different convective heat-transfer behavior of laminar and turbulent flows [2,3]. Treating the test model as a thin-film sensor, the convective heat-transfer coefficient h (or equivalently the Stanton number St) is reconstructed and, through Reynolds analogy [4], it gives information about boundary-layer transition and separation.
Results show a strong link between boundary layer behavior, thermal response and blowing. Normalized temperature trends reveal that the temperature rise is generally lower in the Jet-On case. This is caused by the higher velocities that occur in this case since the flow remains attached, promoting higher shear stress $\tau_w$ on the wall and therefore higher h and lower temperature. In fact, the chordwise St distribution highlights that, with Jet-On, transition is shifted downstream, and flow separation is delayed by keeping the flow attached and a laminar boundary layer over most of the chord. Conversely, Jet-Off exhibits earlier transition and a separated region near the trailing edge. Overall, this study demonstrates IRT as a robust and scalable technique to investigate the effects of circulation control on transition and separation in high-lift airfoils.
$\textbf{References}$:
1. KC Peter, High-lift systems on commercial subsonic airliners, NASA CR-4746, Sept, 1996.
2. Giovanni Maria Carlomagno and Gennaro Cardone, Infrared thermography for convective heat transfer measurements, Experiments in fluids, 49(6):1187–1218, 2010.
3. William Davis and Nicholas R Atkins, Infrared thermography techniques for boundary layer state visualisation, Experiments in Fluids, 65(6):91, 2024.
4. Arthur D. Woodworth, David M. Salazar, and Tianshu Liu, Heat transfer and skin friction: beyond the Reynolds analog, International Journal of Heat and Mass Transfer 206 (2023), p. 123960.
