Artificial Temperature-Difference Enhancement Approaches for Transition Detection with Temperature-Sensitive Paint in European Transonic Windtunnel

The natural-laminar-flow (NLF) technology is a highly promising method to increase the aerodynamic performance of next-generation commercial transport aircraft, while simultaneously reducing their fuel consumption and [Formula: see text] emissions. For an optimal design, detailed knowledge on the boundary-layer evolution over the wings during different steady and dynamic flight conditions is crucial. Temperature-sensitive paint (TSP) is a well-suited optical measurement technique to obtain quantitative information on the spanwise-distributed transition location in model tests at cryogenic temperatures and elevated pressures. For precise boundary-layer transition detection, TSP requires an artificial enhancement of the temperature difference between the laminar and turbulent sections of the boundary layer. This paper presents TSP measurements on a forward-swept wing fuselage-belly fairing configuration, conducted in the European Transonic Windtunnel (ETW), for laminar–turbulent transition detection on the suction side of the wing at various static pitch angles and flight-relevant flow conditions. Three different methods to achieve the necessary temperature difference between the TSP-coated model surface and the flow were tested, and their results were evaluated for their applicability and efficiency in cryogenic testing with TSP. With each method, the transition pattern and its dependency on the pitch angle could be captured successfully with close agreement of the corresponding results.