  The complex flow features resulting from the laminar-turbulent transition
(LTT) in a sudden expansion pipe flow, with expansion ratio of 1:2 subjected to
an inlet vortex perturbation is investigated by means of direct numerical
simulations (DNS). It is shown that the threshold for LTT described by a power
law scaling with -3 exponent that links the perturbation intensity to the
subcritical transitional Reynolds number. Additionally, a new type of
instability is found within a narrow range of flow parameters. This instability
originates from the region of intense shear rate which is a result of the flow
symmetry breakdown. Unlike the fast transition, usually reported in the
literature, the new instability emerges gradually from a laminar state and
appears to be chaotic and strongly unsteady. Additionally, the simulations show
a hysteresis mode transition due to the reestablishment of the recirculation
zone in a certain range of Reynolds numbers. The latter depends on (i) the
initial and final quasi-steady states, (ii) the observation time and (iii) the
number of intermediate steps taken when increasing and decreasing the Reynolds
number.
