History and Progress of Boundary-Layer Transition on a Mach-6 Flared Cone

Results from experimental, theoretical, and computational efforts are combined in a comprehensive review to summarize the improved understanding of the underlying physical mechanisms causing second-mode dominated laminar-turbulent transition on a 3 m circular-arc flared cone for the Boeing/U.S. Air Force Office of Scientific Research Mach 6 Quiet Tunnel (BAM6QT) conditions. Qualitatively similar patterns of streamwise streaks have been observed in experiments, using temperature sensitive paint, and in high-resolved controlled breakdown direct numerical simulations (DNS). The experimental and computational results using nonlinear parabolized stability equations (NPSEs) and DNS revealed that the maximum second-mode pressure fluctuation reaches extremely large values (30-60% of the surface pressure) before the breakdown to turbulence. These values are significantly higher as compared to what has been observed in incompressible flow, therefore providing an explanation for an elongated transition/breakdown regime in high-speed compressible flow. A sudden spectral broadening in the disturbance content has been observed in the three different approaches once the primary wave saturates nonlinearly, indicating a sudden nonlinear amplification of a broad spectrum of frequencies and wave numbers when transition begins. Good agreement was found between NPSE calculations and experiments when comparing the spectral broadening onset location, the total disturbance amplitude, and the amplitude ratio between the primary disturbances and the first harmonics, which shows that NPSEs can accurately reproduce/predict the experimental results. The DNS results revealed that the streaks on the surface of the cone are caused by steady streamwise vortices that are generated by nonlinear interaction of the primary and secondary disturbance wave. The similarities in the streak patterns suggest that the mechanisms observed in the controlled breakdown DNS are likely the same mechanisms leading to laminar-turbulent transition in the natural transition scenario observed in the BAM6QT. The more in-depth understanding of the transition process, and especially the understanding of the mechanisms leading to the streak patterns, can be used to explore potential flow control strategies to mitigate the detrimental effects of the massive heat overshoots over the turbulent values arising as a consequence of these streaks.