Abstract
Hypersonic boundary layers exhibit diverse transition pathways, influenced by
various flow conditions and environments. Non-modal mechanisms, such as the
lift-up effect, are recognized as pivotal contributors to transition,
particularly over complex geometries like rough walls or blunted forebodies. In
this study, we investigate the impact of steady streaks on the transition to
turbulence in hypersonic boundary layers. Streaky baseflows are generated using
optimal disturbances at the inlet, forming the basis for our analysis. We
conduct linearized direct numerical simulations on these baseflows at Mach
number $M_\infty=6.0$, using white noise forcing to trigger instabilities. An
efficient extension of the SPOD method is applied to identify and track the
instabilities originating from the broadband forcing on the 3D non-homogeneous
baseflows. The results highlight the dominant influence of low-frequency
first-mode instabilities and streak-associated instabilities in the linear
regime. These findings emphasize the critical role of streaks in hypersonic
boundary layer transition and provide valuable insights into this complex
phenomenon.