  The 2D second-mode is a potent instability in hypersonic boundary layers
(HBLs). We study its linear and nonlinear evolution, followed by its role in
transition and eventual breakdown of the HBL into a fully turbulent state.
Linear stability theory (LST) is utilized to identify the second-mode wave
through FS-synchronization, which is then recreated in linearly and nonlinearly
forced 2D direct numerical simulations (DNSs). The nonlinear DNS shows
saturation of the fundamental frequency, and the resulting superharmonics
induce tightly braided ``rope-like'' patterns near the generalized inflection
point (GIP). The instability exhibits a second region of growth constituted by
the fundamental frequency, downstream of the primary envelope, which is absent
in the linear scenario. Subsequent 3D DNS identifies this region to be crucial
in amplifying oblique instabilities riding on the 2D second-mode ``rollers''.
This results in lambda vortices below the GIP, which are detached from the
``rollers'' in the inner boundary layer. Streamwise vortex-stretching results
in a localized peak in length-scales inside the HBL, eventually forming haripin
vortices. Spectral analyses track the transformation of harmonic peaks into a
turbulent spectrum, and appearance of oblique modes at the fundamental
frequency, which suggests that fundamental resonance is the most dominant
mechanism of transition. Bispectrum reveals coupled nonlinear interactions
between the fundamental and its superharmonics leading to spectral broadening,
and traces of subharmonic resonance as well.
