Boundary-Layer Transition Detection in a Cryogenic Wind Tunnel Using Luminescent Paint

The objective of the present study was to develop a new technique for detecting the boundary-layer transition. This technique, in which luminescent paints, a digital camera, and digital image processing techniques are used, makes it possible to visualize thermal signatures across the boundary-layer transition. To demonstrate the feasibility of this technique, airfoil models coated with a luminescent paint were tested in the transonic cryogenic wind tunnel. TSP (Temperature-Sensitive Paints), having a strong temperature dependence of the luminescent intensity, were coated on the airfoil models with different thermal insulation properties. The surface temperature difference between laminar and turbulent boundary-layer regions was augmented by a rapid change in liquid nitrogen flow rate in the cryogenic wind tunnel. Luminescent images on the models were taken and the transition images were obtained using the digital image processing technique. As a result, it was found that the boundary-layer transition can be detected using the TSP technique over a cryogenic temperature range from 90 K to 150 K in subsonic and transonic flows.