Flow visualization in a low-density plasma channel

Abstract

A schlieren system and surface-stress-sensitive film system were developed for a plasma channel which posed unique challenges for flow visualization because of the combination of low air density and the presence of plasma discharges. Temperature-sensitive paint and direct-current discharge were also applied to flow visualization. Three pulsed schlieren light sources were evaluated. A light-emitting diode (LED), a xenon NanopulserTM and laser breakdown, were tested on identical flowfields. The LED provided excellent illumination, with pulses ranging from μs to continuous. The NanopulserTM provided excellent, short-duration images, although illumination varied from shot-to-shot. Laser-breakdown provided short-duration, incoherent illumination that was constant from pulse-to-pulse. The surface-stress-sensitive film was applied to surface flow visualization. A low-modulus elastomer doped with a luminescent dye was used to visualize the surface shear stress and pressure field in laminar shock boundary layer interactions. Intensity distributions from the dye were imaged to interrogate the surface pressure gradients. Displacement of surface markers provided shear information. Results showed the presence of Gortler vortices in the reattaching shear flow. Gortler vortices were also evident in temperature-sensitive paint images and in the plasma discharge glow. These vortices were evident in the intensity images from the elastomer, which could be related to the surface pressure gradient, but were not readily evident in surface shear measurements.