Highly-symmetric travelling waves in pipe flow

TL;DR

This paper introduces new highly-symmetric travelling wave solutions in pipe flow, revealing their properties, lower Reynolds number appearance, and potential role in flow transition dynamics.

Contribution

It reports two new classes of symmetric travelling waves and a novel mirror-symmetric wave family without shift-&-reflect symmetry, expanding understanding of flow structures.

Findings

Highly-symmetric waves appear at lower Reynolds numbers.

New wave classes exhibit distinctive structural features.

Waves have higher wall shear stresses than previous solutions.

Abstract

The recent theoretical discovery of finite-amplitude travelling waves in pipe flow has re-ignited interest in the transitional phenomena that Osborne Reynolds studied 125 years ago. Despite all being unstable, these waves are providing fresh insight into the flow dynamics. Here we describe two new classes of highly-symmetric travelling waves (possessing rotational, shift-&-reflect and mirror symmetries) and report a new family of mirror-symmetric waves which is the first found in pipe flow not to have shift-&-reflect symmetry. The highly-symmetric waves appear at lower Reynolds numbers than the originally-discovered non-mirror-symmetric waves found by Faisst & Eckhardt 2003 and Wedin & Kerswell 2004 and have much higher wall shear stresses. The first M-class comprises of the various discrete-rotationally-symmetric analogues of the mirror-symmetric wave found in Pringle & Kerswell (2007) and have a distinctive double layer structure of fast and slow streaks across the pipe radius. The second N-class has the more familiar separation of fast streaks to the exterior and slow streaks to the interior and looks the precursor to the class of non-mirror-symmetric waves already known.