Direct Numerical Simulations of Hypersonic Boundary-Layer Transition for a slender cone

Direct Numerical Simulations (DNS) were carried out to investigate the laminar-turbulent transition process for a slender (2.5◦ half-angle) straight (right) cone at Mach 6 at zero angle of attack. The slender cone geometry of the experiments in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University was used for the numerical investigations. The simulation results indicate that the so-called fundamental breakdown was the dominant nonlinear mechanism in the downstream part of the slender cone geometry where “hot” streaks have recently been observed in the BAM6QT experiments. Contours of the time-averaged Stanton number obtained from DNS also exhibited the formation of “hot” streaks similar to the ones detected in the experiments. The fact that the spacing of the streaks in the azimuthal direction measured in the experiments was comparable to the streak spacing from DNS is further evidence that the fundamental breakdown may indeed be the relevant nonlinear mechanism responsible for the streak development in the BAM6QT experiments. This is consistent with previous findings for a 7◦ half angle straight cone and for a flared cone where DNS also showed that the fundamental breakdown was the dominant nonlinear mechanism. For the flared cone geometry “hot” streaks were also found in both experiment and DNS. The results for the slender cone provide additional evidence that the laminar turbulent transition process and the associated “hot” streak development for the “quiet” flow conditions in the BAM6QT facility is dominated by the fundamental breakdown mechanism.