Vortexons in axisymmetric Poiseuille pipe flows

TL;DR

This paper investigates the nonlinear dynamics of small disturbances in axisymmetric Poiseuille pipe flows, revealing vortexon formations, their splitting behavior, and potential links to flow transition phenomena at high Reynolds numbers.

Contribution

It introduces a reduced KdV model for pipe flow disturbances, identifies vortexon solutions, and explores their evolution and instability, connecting to flow transition mechanisms.

Findings

Vortexons are localized toroidal vortices near pipe boundaries or axis.

Initial vortical patches split into vortexons and vorticity patches.

Vortexon instability may lead to flow transition to puffs and slugs.

Abstract

We present a study on the nonlinear dynamics of small long-wave disturbances to the laminar state in non-rotating axisymmetric Poiseuille pipe flows. At high Reynolds numbers, the associated Navier-Stokes equations can be reduced to a set of coupled Korteweg-de Vries-type (KdV) equations that support inviscid and smooth travelling waves numerically computed using the Petviashvili method. In physical space they correspond to localized toroidal vortices concentrated near the pipe boundaries (wall vortexons) or that wrap around the pipe axis (centre vortexons), in agreement with the analytical soliton solutions derived by Fedele (2012). The KdV dynamics of a perturbation is also investigated by means of an high accurate Fourier-based numerical scheme. We observe that an initial vortical patch splits into a centre vortexon radiating patches of vorticity near the wall. These can undergo further splitting leading to a proliferation of centre vortexons that eventually decay due to viscous effects. The splitting process originates from a radial flux of azimuthal vorticity from the wall to the pipe axis in agreement with the inverse cascade of cross-stream vorticity identified in channel flows by Eyink (2008). The inviscid vortexon most likely is unstable to non-axisymmetric disturbances and may be a precursor to puffs and slug flow formation.