EXPERIMENTAL STUDY ON BOUNDARY LAYER TRANSITION OF ELLIPTICAL CONE CENTERLINE UNDER THE EFFECT OF ANGLE OF ATTACK

The change in angle of attack is one of the common flight variables under real flight conditions, and the resulting changes in flow structure, the distribution of heat and force, and other characteristics cannot be ignored. To investigate the effect of angle of attack on the three-dimensional boundary layer transition related to streamwise vortex instability, an elliptical cone model with a long to short axis ratio of 4:1 is selected. Wind tunnel experiments are conducted to study the mechanism of boundary layer transition on the windward centerline and its variation with angle of attack. Using Temperature Sensitive Paint (TSP) technology and Nano-tracer based Planar Laser Scattering (NPLS) technology, temperature rise curves along the streamwise direction and fine structure images of the boundary layer in the streamwise and transverse directions are obtained at different angles of attack. The transition starting point, disturbance wave characteristic frequency, and the variation law of fine boundary layer structure with angle of attack are analyzed. Based on the characteristic frequency and amplitude information of boundary layer disturbance waves obtained from pulsating pressure testing, the obtained laws are further demonstrated and explained. The main conclusions are as follows: when the angle of attack varies from 0° to 2° , the boundary layer of the elliptical cone centerline undergoes an evolution from laminar to turbulence. The transition process of the boundary layer is dominated by low-frequency characteristic frequencies, and the disturbance characteristic frequency increases with the increase of angle of attack but the amplitude remains unchanged. The starting point of transition is not sensitive to the changes in angle of attack. However, when the angle of attack increases to 5°, the boundary layer can maintain stable growth of disturbance waves within the observation range, and the transition is significantly delayed without the boundary layer evolving to the turbulence stage.