A random forcing approach was implemented into a high-order accurate finite difference code in order to investigate “natural” laminar-turbulent transition in hypersonic boundary layers. In hypersonic transition wind-tunnel experiments, transition is caused “naturally” by free-stream disturbances even when so-called quiet tunnels are employed such as the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The nature and composition of the free-stream disturbance environment in high-speed transition experiments is difficult to assess and therefore largely unknown. Consequently, in the Direct Numerical Simulations (DNS) presented here, the free-stream disturbance environment is simply modeled by random pressure (acoustic) disturbances with a broad spectrum of frequencies and a wide range of azimuthal wave numbers. Results of a high resolution DNS for a flared cone at Mach 6, using the random forcing approach, are presented and compared to a fundamental breakdown simulation using a “controlled” disturbance input (with a specified frequency and azimuthal wave number). The DNS results with random forcing clearly exhibit the “primary” and “secondary” streak pattern, which has previously been observed in our “controlled” breakdown simulations and the experiments in the BAM6QT. In particular, the spanwise spacing of the “primary” streaks for the random forcing case is identical with the spacing obtained from the “controlled” fundamental breakdown simulation. A comparison of the wall
pressure disturbance signals between the random forcing DNS and experimental data shows remarkable
agreement. The random forcing approach seems to be a promising strategy to investigate nonlinear
breakdown in hypersonic boundary layers without introducing any bias towards a distinct nonlinear
breakdown mechanism and/or the selection of specific frequencies or wave numbers that is required in the
“controlled” breakdown simulations.
