Characterization of Transitional, High-Enthalpy Boundary Layers on a Slightly-Blunted Cone. Part I: Schlieren Imaging

This paper presents an experimental investigation into the boundary-layer stability of a slender cone in hypervelocity flow within the T5 reflected-shock tunnel. Schlieren imaging was utilized to characterize the frequency content and disturbance structures experienced within the boundary layer of a 5◦ cone in a Mach-5 freestream with high reservoir enthalpy, 8-10 MJ/kg. The effects of varying freestream Reynolds number and nose bluntness were examined. Second-mode frequency peaks between 1200-1300 kHz were identified in all cases, but they persisted over a longer extent in the sharper-nose cases. The bluntest nosetip case exhibited unique nonmodal structures which extended into the freestream, and the signature of the second-mode appeared limited to the near-wall region. N factors contours suggested interplay between second-mode content and frequency content outside the second-mode band. Cross-bicoherence calculations indicated that, for the sharper nosetip, nonlinear interactions between the second-mode fundamental and its first harmonic persisted to the point of breakdown. For the blunter nosetip, dominant nonlinear interactions involved low-frequency content.