Optical Measurements of Transitional Events in a Mach-6 Boundary Layer

Results of aero-optical measurements of naturally occurring transitional structuresturbulent spots and second-mode wave packetsin a hypersonic laminar boundary layer are presented. Optical spectra of turbulent spots were similar to spectra for fully turbulent boundary layers, and the average thickness of the boundary layer during the turbulent spot was approximately 50% of the fully turbulent boundary layer. The convective speed of the leading edge of the spot was measured to be 0.95 of the freestream speed, whereas the trailing edge moves at 0.7 of the freestream speed. The dominant frequency of the wave packet structures was about 70 kHz, consistent with the theoretically predicted second-mode frequency; the average convective speed of the packets was measured to be 0.88 of the freestream speed. Using the frozen field hypothesis, the wave fronts were stitched together to reveal streamwise/spanwise topology of the packets. The packets were found to be fairly narrow, about 2-3 in the spanwise direction, extending for 30-60 in the streamwise direction.