Application of High-Speed Pressure Sensitive Paint to Low-Speed Vortical Flow at High Angles of Attack

Abstract

Highly maneuverable aircraft configurations create complex vortex systems with vortex breakdown at moderate subsonic speeds and medium to high angles of attack. High levels of pressure fluctuations on the configuration surface can be observed downstream of vortex breakdown. These fluctuations can cause significant structural vibrations known as tail buffeting. Therefore, the detailed measurement of these pressure fluctuations and the corresponding vibrations is of great importance in aircraft aerodynamics and aeroelasticity. Pressure Sensitive Paint (PSP) is an established tool to measure steady surface pressures without installation of a great number of pressure sensors in the model. Fast-response pressure sensitive paint (iPSP) was developed at the German Aerospace Center (DLR) to measure unsteady pressure fluctuation effects in a setup with high-speed cameras. The setup allows a high spatio-temporal resolution of the unsteady surface pressures. To the authors’ knowledge fast response pressure sensitive paint has not been applied to low-speed vortical flows at high angles of attack yet. In the present paper the iPSP method is applied to a mixed-material, flexible aircraft wind tunnel model in these flow conditions. The configuration represents a modern highly maneuverable aircraft with double-delta wing and horizontal tail plane. The low-speed buffeting effects on the configuration are analysed using a synchronised system of five connected high-speed cameras to simultaneously measure the unsteady surface pressures with iPSP and the dynamic three-dimensional structural deformation. The present application case is particularly challenging due to low levels of pressure fluctuation in the low-speed regime, multi-frequzency aeroelastic phenomena on the model, a long distance between the cameras and the model, as well as the combination of synchronized masurement of unsteady surface pressure and structural deformation. Advanced data analysis methods are applied in a post-processing too chain to cope with these challenges. The results for the root-mean square surface pressures show good agreement with high-fidelity CFD results for the flow conditions of interest. Particular focus of the analysis is on the lessons learned regarding the UV illumination setup, the camera arrangement as well as post-processing tools.