Control of hypersonic boundary layers by helium injection

Results are presented on the control of a zero-pressure gradient Mach 8 boundary layer using low-momentum helium gas injected through a transverse slot. Planar Filtered Rayleigh Scattering and Sodium Laser-Induced Fluorescence were used to study the flow. The boundary layer structure was visualized with and without helium injection in three orthogonal planes. Using a new MHz imaging system, short movies were obtained showing the evolution of the boundary layer downstream of the slot. These sequential images were used to construct volumetric representations of the instantaneous boundary layer behavior. The results indicate that low levels of helium injection can have a substantial effect on the boundary layer structure. Highly organized longitudinal structures develop downstream of the slot and persist for long distances downstream, remaining fixed in their spanwise position. They appear to be streamwise vortices, and it is speculated that they are formed near the injection slot due to local concave streamline curvature.