Effects of transverse helium injection on hypersonic boundary layers

Results are presented on the control of a zero-pressure gradient Mach 8 bou ndary layer using low-momentum helium injection through a transverse slot. The boundary layer structure was visualized in three orthogonal planes usin g planar filtered Rayleigh scattering. Sequential images of the boundary la yer were obtained using a new MHz imaging system and employed to construct three-dimensional representations of the instantaneous boundary layer behav ior. The results indicate that low levels of helium injection into transiti onal and turbulent boundary layers can have a substantial effect on the bou ndary layer structure. Longitudinal structures develop downstream of the sl ot 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 a pplications. (C) 2001 American. Institute of Physics.