Effects of transverse helium injection on hypersonic boundary layers

Results are presented on the control of a zero-pressure gradient Mach 8 boundary layer using low-momentum helium injection through a transverse slot. The boundary layer structure was visualized in three orthogonal planes using planar filtered Rayleigh scattering. Sequential images of the boundary layer were obtained using a new MHz imaging system and employed to construct three-dimensional representations of the instantaneous boundary layer behavior. The results indicate that low levels of helium injection into transitional and turbulent boundary layers can have a substantial effect on the boundary layer structure. Longitudinal structures develop downstream of the slot and persist for long times. They appear to be streamwise vortices, and it is speculated that they are formed near the injection slot due to local streamline concave curvature. The results may be important for aero-optic applications.