Time-Resolved Schlieren Analysis of Boundary-Layer Transition on a Blunt Cone in Hypersonic Flow

This work presents a time-resolved Schlieren investigation of boundary-layer transition over a blunt cone in hypersonic flow, performed in the VKI Longshot wind tunnel and synchronized with high-frequency wall-pressure measurements. Experiments are conducted at free-stream Mach numbers between 10.5 and 12 and unit Reynolds numbers ranging from 3 × 106 m−1 to 11 × 106 m−1. Three configurations featuring nosetip radii of 1.75 mm, 4.75 mm, and 10 mm are investigated to explore different instability regimes associated with bluntness effects. The implementation of high-speed Schlieren imaging at the acquisition frequency of 620 kHz enables continuous time-resolved visualization of instability-wave evolution throughout the entire test duration. Cross-correlation analyses of consecutive Schlieren frames provide estimates of the instability convection velocity, which are compared with values obtained from synchronized wall-pressure measurements and Linear Stability Theory (LST) predictions. Good agreement is observed between the different techniques, demonstrating the capability of the optical diagnostics to provide quantitative information on instability propagation. For the slightly blunt configuration, clear rope-like structures associated with second-mode instabilities are captured within the boundary layer. Frequency spectra obtained from Schlieren intensity distributions and wall-pressure measurements exhibit dominant peaks between 150 and 300 kHz, consistently evolving with the free-stream Reynolds number and boundary-layer thickness. For the moderately blunt configuration, transition is delayed and intermittent hairpin-like structures appear (strongly differing from second mode waves and characteristic of entropy layer instabilities) within a predominantly laminar boundary layer. The highly blunt configuration exhibits larger-scale structures developing within the entropy layer and qualitatively resembling the hairpin structures detected for the moderately blunt configuration. This suggests the emergence of instability mechanisms different from classical second-mode waves and the possible onset of the transition reversal regime. The present results demonstrate the capability of time-resolved Schlieren imaging to characterize hypersonic boundary-layer instabilities quantitatively and provide new experimental insight into the effects of nose bluntness on transition dynamics in hypersonic flows.