Implementation of a Laser-Based Schlieren System for Boundary Layer Instability Investigation in the VKI H3 Hypersonic Wind Tunnel

摘要

The work presented in this study is a step towards being able to measure the effects of ablation-induced roughness on boundary-layer transition. Due to material limitations such as thermal conductivity or fragility standard methods of locating the transition onset, for example infrared thermograph} or surface-mounted pressure probes, cannot always be used. However schlieren imagery is not affected by these issues. To take advantage of this, a laser-based schlieren setup has been installed on the VKI H3 hypersonic wind tunnel. This schlieren system allows for the determination of boundary-layer thickness, and second-mode instability properties such as their convection velocity, wavelengths, and frequencies. This is achieved by capturing high-resolution, frozen-flow images of the boundary layer. Comparisons between the schlieren data, linear stability theory calculations, and previous tests done in the VKI H3 wind tunnel on a smooth 7° half-angle cone show good agreement. The technique was then applied to characterize the boundary layer over two graphite cones. One of which had been ablated in the VKI Plasmatron high-enthalpy wind tunnel and one reference. This characterization is unachievable with IR thermography or high-speed pressure sensors due to the aforementioned material limitations. The schlieren technique, however, was able to characterize the boundary layer once instabilities were visible. With this addition, the suite of measurement techniques for the VKI H3 hypersonic wind tunnel has been meaningfully expanded. These achievements are a step towards the accurate characterization of ablation induced hypersonic transition phenomena.