Direct Numerical Simulations of the Nonlinear Boundary Layer Transition Regime on a Flat Plate at Mach 6

Abstract

Direct Numerical Simulations (DNS) were carried out in order to investigate the laminar-turbulent transition process of a hypersonic boundary-layer on a flat plate. The conditions of the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University were used for the simulations. Results from a three-dimensional nonlinear wave packet simulation indicated that all of the ``classical'' nonlinear mechanisms (fundamental resonance, subharmonic resonance, oblique breakdown) are possible breakdown scenarios for a hypersonic boundary-layer on a flat plate. A detailed investigation of the secondary instability regime showed that the fundamental resonance was much stronger than the subharmonic resonance. High-fidelity ``controlled'' breakdown simulations for first mode oblique and second mode fundamental, exhibited the development of ``hot'' streak patterns on the surface of the flat plate. Streamwise ``hot'' streaks were also observed in the flared cone experiments at the BAM6QT facility and in DNS of high-speed transition for straight and flared cones. For the flat plate the fundamental resonance and the oblique breakdown mechanisms lead to a breakdown to turbulence. The transition region of the second mode fundamental breakdown is longer than for the first mode oblique breakdown.