Influence of free stream disturbance on hypersonic boundary layer dynamics over a cone–cylinder–flare

Hypersonic transition studies on systems sustaining multimodal dynamics are critical to understanding aerothermal loading on flight-relevant configurations. The present work evaluates transition mechanisms in hypersonic boundary layers over a cone–cylinder–flare geometry, and its sensitivity to free stream disturbance amplitudes, using a global linear stability approach and direct numerical simulations (DNS). Under relatively quiet conditions, the flow field resembles the laminar solution, consisting of a large separation zone over the cylinder–flare junction. Linear analysis identifies multiple convective instabilities including, oblique first modes and two-dimensional second modes over the cone segment, and shear layer instabilities over the separation zone. This separation zone also supports a stationary global instability, producing streamwise streaks with an azimuthal wavenumber, $m=21$ , which eventually drives transition as captured in the DNS. Conversely, at higher disturbance amplitudes, the largely attached boundary layer transitions through a bypass mechanism, involving intermodal interactions between low-frequency streaks, and first mode instabilities. The resulting upstream shift in transition onset leads to a significant rise in both steady and unsteady surface loading. Peak thermal loading under quiet conditions displays the signature of the linear global instability over the flare, whereas that under noisier environments is dominated by an imprint of unsteady Görtler vortices over the cylinder–flare junction.