Implementation of Self-Aligned Focusing Schlieren for Hypersonic Boundary Layer Measurements

Abstract

View Video Presentation: https://doi.org/10.2514/6.2023-2438.vid A self-aligned focusing schlieren (SAFS) apparatus based on a recently published design is implemented for the purpose of making near-planar density gradient measurements of hypersonic boundary layers. At a working distance of one meter, the current SAFS setup is shown to be capable of depths of focus on the order of 1-10 mm while also achieving framerates in the 100 kHz to 1+ MHz range at exposures as short as 20 ns. These qualities are achieved while remaining resistant to tunnel vibrations due to the nature of the retroreflected beam path. The design space of the apparatus is also investigated and reveals the influence of key components/parameters on the overall system performance and capabilities. The design is found to be extensible to a variety of scales that may prove applicable to a wide range of research facilities. Finally, a series of experiments were executed in the Air Force Research Laboratory Mach-6 Ludwieg Tube over a variety of axisymmetric and non-axisymmetric geometries using the SAFS setup to capture images of the developing boundary layer and passing flow disturbances. Comparisons between SAFS and traditional, line-of-sight schlieren are presented alongside discussions on the overall performance compared to the predicted design space.