Dynamics of propagating turbulent pipe flow structures. Part II: Relaminarization

TL;DR

This study investigates the relaminarization process in turbulent pipe flow, revealing the decay of high-frequency modes, the chugging cycles during transition, and the outer layer's role in turbulence sustenance.

Contribution

It provides detailed insights into the dynamics of flow relaminarization and the role of different flow structures using Karhunen-Loève decomposition and direct numerical simulation.

Findings

High frequency modes decay first during relaminarization.

Flow undergoes three chugging cycles before fully relaminarizing.

Outer layer modes decay first, highlighting their role in turbulence maintenance.

Abstract

The dynamical behavior of propagating structures, determined from a Karhunen-Lo`eve decomposition, in turbulent pipe flow undergoing reverse transition to laminar flow is investigated. The turbulent flow data is generated by a direct numerical simulation started at a fully turbulent Reynolds number of Re_\tau=150, which is slowly decreased until Re_\tau=95. At this low Reynolds number the high frequency modes decay first, leaving only the decaying streamwise vortices. The flow undergoes a chugging phenomena, where it begins to relaminarize and the mean velocity increases. The remaining propagating modes then destabilize the streamwise vortices, rebuild the energy spectra, and eventually the flow regains its turbulent state. Our results capture three chugging cycles before the flow completely relaminarizes. The high frequency modes present in the outer layer decay first, establishing the importance of the outer region in the self-sustaining mechanism of wall bound turbulence.