Experiments are performed in a Mach-6 shock tunnel to examine the laminar-to-turbulent transition process associated with a sudden increase in surface angle on a slender body. A cone/flare geometry with a 5$^\circ$ frustum and compression angles ranging from 5$^\circ$ to 15$^\circ$ allow a range of mean flow configurations, spanning an attached shock-wave/boundary-layer interaction to a fully separated one; the unit Reynolds number of the flow is also varied to modify the state of incoming second-mode boundary-layer disturbances. Ultra-high-speed schlieren visualizations provide a global picture of the flow development, supplemented by high-frequency surface pressure measurements. For the 5$^\circ$ compression, the unsteady flow field is dominated by the second-mode waves, whose breakdown to turbulence is generally accelerated (compared with the straight-cone configuration) by encountering the angle change. As the compression angle is increased to induce separation, lower-frequency disturbances appear along the separated shear layer that exhibit much larger amplification rates than the incoming second-mode waves; the latter effectively freeze in amplitude downstream of the separation point before rapidly breaking down upon reattachment. The shear-layer disturbances become dominant at the largest compression angle tested. Radiation of disturbance energy to the external flow is consistently observed: this generally occurs along mean flow features (flare, separation or reattachment shocks) for the second-mode disturbances and spontaneously for the shear-layer waves. The combined application of spectral proper orthogonal decomposition and a global bispectral analysis allows the identification of important unsteady flow structures and the association of these with prominent nonlinear interactions in the various configurations.

An experimental campaign was conducted to examine the impact of an abrupt change in surface geometry on hypersonic boundary-layer instability waves. The primary test configuration consisted of a $5^{\circ }$ half-angle, nominally sharp cone with a $15^{\circ }$ half-angle flare attachment. Tests were conducted at Mach 6 with the unit Reynolds number varying from $3.0\times 10^6$ to $4.9\times 10^6\ \textrm {m}^{-1}$. The $10^{\circ }$ compression was sufficient to create a small separation region at the cone–flare junction at these conditions. Ultra-high-speed schlieren (822 kHz) revealed the propagation of second-mode disturbances with frequencies between 200 and 300 kHz within the upstream boundary layer; when these reached the separation region, radiation of disturbance energy along the separation shock was observed. Tests conducted at low unit Reynolds numbers demonstrated inhibited instability growth (compared to the straight-cone case) through the separation region and the development of low-frequency (${\sim }75\ \textrm {kHz}$) instabilities within the separated shear layer. At higher Reynolds numbers, however, the corner interaction was found to cause rapid breakdown near reattachment, leading to earlier transition than for a straight cone. Analysis of the schlieren images using spectral proper orthogonal decomposition provided a global picture of the structure and development of the second-mode and shear-generated instabilities.