  We study the transition to turbulence in a flat plate boundary layer by means
of visibility analysis of velocity time-series extracted across the flow
domain. By taking into account the mutual visibility of sampled values,
visibility graphs are constructed from each time series. The latter are, thus,
transformed into a geometrical object, whose main features can be explored
using measures typical of network science that provide a reduced order
representation of the underlying flow properties. Using these metrics, we
observe the evolution of the flow from laminarity to turbulence and the effects
exerted by the free-stream turbulence. Different from other methods requiring
an extensive amount of spatiotemporal data (e.g., full velocity field) or a set
of parameters and thresholds arbitrarily chosen by the user, the present
network-based approach is able to identify the onset markers for transition by
means of the streamwise velocity time-series alone.
