Visualization of Görtler vortices in high-enthalpy plasma flow boundary layer on compression ramp using planar laser-induced fluorescence

The Görtler vortex is a characteristic flow feature observed in the boundary layer on compression ramp in hypersonic flow. In the context of high-enthalpy plasma flows during aerospace re-entry processes, there is currently a lack of effective means to visualize the boundary layer. In this study, the Nitric Oxide Planar Laser-Induced Fluorescence (NO-PLIF) technique was employed to visualize the boundary layer of a compression ramp in a 50-MW arc-heated plasma wind tunnel. Görtler-like vortex structures were observed in the boundary layer of the ramp. This is the first time that Görtler vortices have been clearly observed in a high-enthalpy plasma flow. By varying the flow conditions, the Görtler vortices persisted in the boundary layer of the ramp when the total enthalpy of the arc-heated wind tunnel exceeded 12.3 MJ/kg. Several image processing techniques were applied to extract the structure of high-speed Görtler streaks, and the position of the high-speed streaks was found to be non-fixed, whereas the average Görtler wavelength remained at approximately 30 mm at a 10° ramp and showed limited variation with the total enthalpy. Additionally, a sheet-forming optics system with an adjustable angle and height was designed to enable visualization of the Görtler vortices in the boundary layer of the ramp at different angles and heights. The vortices on the low-angle ramp exhibited better stability and shorter wavelengths. Visualization results at different heights confirmed that the Görtler vortex wavelength was approximately twice the boundary layer thickness. This study demonstrates the feasibility and potential of the PLIF technique for the visualization of the boundary layer in plasma flows, especially with regard to Görtler vortices.