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

TL;DR

This study uses direct numerical simulations to explore how vortex perturbations influence localized turbulence in a circular pipe with expansion, revealing the role of perturbation strength and flow structures in turbulence transition.

Contribution

It introduces a vortex perturbation method to analyze turbulence onset in pipe flow with expansion, showing a less abrupt transition and detailed flow structure analysis.

Findings

Localized turbulence depends on vortex perturbation strength.

Recirculation region transitions to turbulence at high Reynolds numbers.

Proper orthogonal decomposition highlights energetic flow near expansion.

Abstract

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.