The spatio-temporal features of the velocity field of a fully-developed
turbulent channel flow are investigated through the natural visibility graph
(NVG) method, which is able to fully map the intrinsic structure of the
time-series into complex networks. Time-series of the three velocity
components, (u,v,w), are analyzed at fixed grid-points of the whole
three-dimensional domain. Each time-series was mapped into a network by means
of the NVG algorithm, so that each network corresponds to a grid-point of the
simulation. The degree centrality, the transitivity and the here proposed mean
link-length were evaluated as indicators of the global visibility,
inter-visibility, and mean temporal distance among nodes, respectively. The
metrics were averaged along the directions of homogeneity (x, z) of the flow,
so they only depend on the wall-normal coordinate, y^+. The visibility-based
networks, inheriting the flow field features, unveil key temporal properties of
the turbulent time-series and their changes moving along y^+. Although
intrinsically simple to be implemented, the visibility graph-based approach
offers a promising and effective support to the classical methods for accurate
time-series analyses of inhomogeneous turbulent flows