  A high-fidelity simulation of the shock/transitional boundary layer
interaction caused by a 15-degrees axisymmetrical compression ramp is performed
at a freestream Mach number of 5 and a transitional Reynolds number. The inlet
of the computational domain is perturbed with a white noise in order to excite
convective instabilities. Coherent structures are extracted using Spectral
Proper Orthogonal Decomposition (SPOD), which gives a mathematically optimal
decomposition of spatio-temporally correlated structures within the flow. The
mean flow is used to perform a resolvent analysis in order to study non-normal
linear amplification mechanisms. The comparison between the resolvent analysis
and the SPOD results provides insight on both the linear and non-linear
mechanisms at play in the flow. To carry out the analysis, the flow is
separated into three main regions of interest: the attached boundary layer, the
mixing layer and the reattachment region. The observed transition process is
dependent on the linear amplification of oblique modes in the boundary layer
over a broad range of frequencies. These modes interact nonlinearly to create
elongated streamwise structures which are then amplified by a linear mechanism
in the rest of the domain until they break down in the reattachment region. The
early nonlinear interaction is found to be essential for the transition
process.
