  Large-eddy simulations of a flat-plate boundary layer, without a leading
edge, subject to multiple levels of incoming free stream turbulence are
considered in the present work. Within an input-output model where non-linear
terms of the incompressible Navier-Stokes equations are treated as an external
forcing, we manage to separate inputs related to perturbations coming through
the intake of the numerical domain, whose evolution represents a linear
mechanism, and the volumetric non-linear forcing due to triadic interactions.
With these, we perform the full reconstruction of the statistics of the flow,
as measured in the simulations, to quantify pairs of wavenumbers and
frequencies more affected by either linear or non-linear receptivity
mechanisms. Inside the boundary layer, different wavenumbers at near-zero
frequency reveal streaky structures. Those that are amplified predominantly via
linear interactions with the incoming vorticity occur upstream and display
transient growth, while those generated by the non-linear forcing are the most
energetic and appear in more downstream positions. The latter feature vortices
growing proportionally to the laminar boundary layer thickness, along with a
velocity profile that agrees with the optimal amplification obtained by linear
transient growth theory. The numerical approach presented is general and could
potentially be extended to any simulation for which receptivity to incoming
perturbations needs to be assessed.
