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The Boeing/AFOSR Mach-6 Quiet Tunnel achieved quiet flow to a stagnation pressure of 163 psia in Dec. 2008, the highest value observed so far. It remains quiet at pressures above 160 psia. Under noisy conditions, nozzle-wall boundary-layer separation and the associated tunnel shutdown appear to propagate slowly upstream, whereas under quiet conditions, the propagation is very rapid. A new diffuser insert has been designed, fabricated, and installed in the tunnel in order to start larger blunt models and increase run time. A flared cone with a circular-arc geometry was designed to generate large second-mode N factors under quiet flow conditions. When the computed N factor was 13, large instability waves were measured under quiet flow conditions using fast pressure sensors, but the flow remained laminar. Transition was observed only under noisy conditions. A laminar instability was detected in the wake of an isolated roughness element in the boundary layer on the nozzle wall; this appears to be the first such measurement at hypersonic speeds.

Surface pressure-sensors have been used to measure the second-mode boundary layer instability on a 7• half-angle sharp cone at zero angle of attack in the quiet Mach-6 wind tunnel at Purdue and in the conventional Mach-6 wind tunnel in Braunschweig. The measurements were made using a stream-wise array of high-frequency sensors. They show the second-mode waves in quiet and noisy flow at different unit Reynolds numbers. The quiet flow data is compared to results in noisy flow. The signal quality allows for the calculation of amplification rates, which are compared to the results of linear stability computations.

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Aerothermodynamics and hypersonic flows involve complex multi-disciplinary physics, including finite-rate gas-phase kinetics, finite-rate internal energy relaxation, gas-surface interactions with finite-rate oxidation and sublimation, transition to turbulence, large-scale unsteadiness, shock-boundary layer interactions, fluid-structure interactions, and thermal protection system ablation and thermal response. Many of the flows have a large range of length and time scales, requiring large computational grids, implicit time integration, and large solution run times. The University of Minnesota / NASA US3D code was designed for the simulation of these complex, highly-coupled flows. It has many of the features of NASA's well-established DPLR code, but uses unstructured grids and has many advanced numerical capabilities and physical models for multi-physics problems. The main capabilities of the code are described, the physical modeling approaches are discussed, the different types of numerical flux functions and time integration approaches are outlined, and the parallelization strategy is overviewed. Comparisons between US3D and the NASA DPLR code are presented, along with several advanced simulations to illustrate some novel features of the code.