  We report the results of three-dimensional direct numerical simulations for
incompressible viscous fluid in a circular pipe flow with a sudden expansion.
At the inlet, a parabolic velocity profile is applied together with a finite
amplitude perturbation in the form of a vortex with its axis parallel to the
axis of the pipe. At sufficiently high Reynolds numbers the recirculation
region breaks into a turbulent patch that changes position axially depending on
the strength of the perturbation. This vortex perturbation is believed to
produce a less abrupt transition than in previous studies with a tilt
perturbation, as the localized turbulence is observed via the formation of a
wavy structure at a low order azimuthal mode, which resembles an optimally
amplified perturbation. For higher amplitude, the localized turbulence remains
at a constant axial position. It is further investigated using proper
orthogonal decomposition, which indicates that the centre region close to the
expansion is highly energetic.
