The results of a seminal experimental study of the effects of bluntness and swallowing length on transition on an 8 deg cone at zero angle of attack in Mach 6 high-Reynolds-number flow are analyzed with the STABL-2D computational fluid dynamics code package. Mean flow solutions and parabolized stability analyses for a total of 11 different nose-tip bluntnesses, ranging from sharp to a 15.24 mm radius, are obtained. For the sharpest cases, the $N$ factor at transition is approximately seven; but, as bluntness increases and the calculated swallowing distance lengthens, the computed $N$ factor at the experimentally observed transition location drops below the level at which Mack’s second mode would be expected to lead to transition. These results indicate that the dominant instability mechanism for the bluntest cases is not the second mode, and oblique mode analysis also excludes these modes as the dominant instability mechanism.

Experiments are performed to investigate the effect of nose-tip bluntness on the instability mechanisms leading to boundary-layer transition on a $7^{\circ }$ half-angle cone in a Mach-6 free stream. The development of disturbances is characterized using a combination of high-speed calibrated schlieren images and pressure measurements, and the data are compared with results computed using the parabolized stability equations. The approximately 414 mm long cone model is equipped with an interchangeable nose tip ranging from sharp to 5.08 mm in radius. For nose tips with a radius $R_{N}<2.54\ {\rm mm}$ , second-mode instability waves are the dominant mechanism leading to transition. Time-averaged frequency spectra computed from the calibrated schlieren visualizations and pressure measurements are used to compute the second-mode most-amplified frequencies and integrated amplification rates ( $N$ factors). Good agreement is observed between the measurements and computations in the linear-growth regime for the sharp-nose configuration at each free-stream condition. Additionally, a bispectral analysis identifies quadratic phase locking of frequency content responsible for the growth of higher harmonics. For nose tips of $R_{N}\geqslant 2.54\ {\rm mm}$ , the schlieren visualization region is upstream of the entropy-layer swallowing length, and second-mode waves are no longer visible within the boundary layer; instead, elongated, steeply inclined features believed to be associated with non-modal instability mechanisms develop between the entropy-layer and boundary-layer edges. Simultaneously acquired surface pressure measurements reveal high-frequency pressure oscillations similar to second-mode instability waves associated with the trailing edge of these non-modal features.

Instability measurements of an axisymmetric, laminar separation bubble were made over a sharp cone-cylinder-flare with a $12^{\circ }$ flare angle under hypersonic quiet flow. Two distinct instabilities were identified: Mack's second mode (which peaked between 190 and 290 kHz) and the shear-layer instability in the same frequency band as Mack's first mode (observed between 50 and 150 kHz). Both instabilities were measured with surface pressure sensors and were captured with high-speed schlieren. Linear stability analysis results agreed well with these measured instabilities in terms of both peak frequencies and amplification rates. Lower-frequency fluctuations were also noted in the schlieren data. Bicoherence analysis revealed nonlinear phase-locking between the shear-layer and second-mode instabilities. For the first time in axisymmetric, low-disturbance flow, naturally generated intermittent turbulent spots were observed in the reattached boundary layer. These spots appeared to evolve from shear-layer-instability wave packets convecting downstream. This work presents novel experimental evidence of the hypersonic shear-layer instability contributing directly to transition onset for an axisymmetric model.

Instabilities generated in Mach-6 quiet flow about an axisymmetric cone-cylinder-flare geometry with a $5^{\circ }$ half-angle sharp cone were studied in the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT). A small, $3.5{}^{\circ }$ flare angle was initially tested to provide a baseline with a model that introduces a compression to the flow but does not induce a separation. A ${10}^{\circ }$ flare was then used to make measurements with an axisymmetric separation bubble. Surface heat flux and pressure fluctuation measurements were made with IR thermography and PCB and Kulite pressure sensors, respectively. Off-the-surface density gradient fluctuations were observed using focused laser differential interferometry for the separated flow. Quiet-flow measurements resulted in laminar boundary layers through the downstream edge of both models. The shear layer was found to be mostly steady in space under quiet flow, as opposed to large spatial fluctuations seen in imagery from conventional noise tunnels. In addition to the second mode, a traveling instability was observed which, based on computational analysis, corresponds to oblique waves that are amplified by the shear layer above the separation bubble. This instability was found to phase lock with the second mode as it propagates downstream. Artificial perturbations were then introduced to the ${10}^{\circ }$ flare model upstream of the separation to study the resulting wave packets as they moved through the shear and boundary layers. This paper provides measurements that may be comparable to hypersonic flight due to the low-disturbance environment of the test facility.