Transition and Tunnel Noise Effects on Three-Dimensional Separated Shear Layers

Experiments investigating three-dimensional separated flows and reattached boundary-layer transition behaviors on a cone/inclined-slice/ramp test article were conducted in the Boeing/AFOSR (The Air Force Office of Scientific Research) Mach-6 Quiet Tunnel at freestream unit Reynolds numbers from [Formula: see text] to [Formula: see text] with low freestream noise levels. Infrared thermography revealed a strong Reynolds-number dependence in the formation and subsequent splitting of streaks within transitional and turbulent reattachment regimes on the ramp. Comparisons between quiet- and conventional freestream noise levels showed that, although conventional-noise results remained markedly different from those in quiet flow, reducing the unit Reynolds number under conventional-noise conditions by a factor of 3 yielded flow features more closely resembling the heat flux and surface pressure fluctuation behavior of quiet-flow experiments than Reynolds-matched cases. Low-frequency/large-scale shear-layer oscillation was observed in surface pressure measurements and was corroborated by spectral analysis of high-speed schlieren videography using power spectral densities, globally averaged magnitude-squared coherence, and spectral proper orthogonal decomposition in both noisy and quiet flow. At Reynolds numbers above approximately [Formula: see text] in quiet flow, the reattached shear layer became intermittently turbulent; laminar periods were observed for time periods under 2 ms and manifested sporadically with no clear dominating period.