Boundary layer transition detection on wind tunnel models during continuous pitch traverse

Abstract

For aerodynamic profile tests on aircraft models, boundary-layer transition detection is of great interest. Under ambient flow conditions the infrared technique (IR) is a well-established image-based method for this purpose. In high Reynolds number tests that are conducted at cryogenic temperatures the IR technique is of only limited suitability. In contrast, the image-based Temperature-Sensitive Paint (TSP) technique is well suited for these conditions. Boundary layer transition detection by means of TSP generally requires an artificial temperature difference between model surface and flow. For wind tunnels operated under cryogenic conditions changing the liquid nitrogen injection rate of the working fluid causing a rapid change of the flow temperature can generate this temperature difference (“temperature-step method”). The drawback of this temperature-step method is that during the change of the flow temperature neither the Reynolds nor the Mach number can be kept constant. To overcome this problem, we have recently published an alternative approach where Carbon Nanotubes (CNT) are used to electrically heat the model surface and thus generate a well-defined temperature difference to visualize laminar-turbulent transition. The combination of CNT and TSP, which we call cntTSP, delivered excellent results in different wind tunnel tests from ambient temperatures down to 100 K. The cntTSP sensor visualizes the surface boundary-layer conditions as temperature distribution based on different heat transfer coefficients in the laminar and turbulent flow. Compared to the temperature-step method the cntTSP approach has the advantage and potential to measure boundary-layer transition in a time-resolved manner, e.g. on wind tunnel models when the wind tunnel is operated in continuous pitch traverse mode, which is desired in order to improve the data productivity of the wind tunnel tests. In this paper, the applicability of cntTSP to a continuous moving model (pitch-traverse or pitch-sweep test) is investigated in the pilot facility of the European Transonic Windtunnel (PETW). The laminar to turbulent boundary-layer transition appearing on a 2D model is visualized and compared between the pitch-traverse and pitch-pause tests.