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We report the results of three-dimensional direct numerical simulations forincompressible viscous fluid in a circular pipe flow with a sudden expansion.At the inlet, a parabolic velocity profile is applied together with a finiteamplitude perturbation in the form of a vortex with its axis parallel to theaxis of the pipe. At sufficiently high Reynolds numbers the recirculationregion breaks into a turbulent patch that changes position axially depending onthe strength of the perturbation. This vortex perturbation is believed toproduce a less abrupt transition than in previous studies with a tiltperturbation, as the localized turbulence is observed via the formation of awavy structure at a low order azimuthal mode, which resembles an optimallyamplified perturbation. For higher amplitude, the localized turbulence remainsat a constant axial position. It is further investigated using properorthogonal decomposition, which indicates that the centre region close to theexpansion is highly energetic.

Flow in a Circular Expansion Pipe Flow: Effect of a Vortex Perturbation on Localized Turbulence