Localized travelling waves in the asymptotic suction boundary layer

TL;DR

This paper discovers two spanwise-localized travelling wave solutions in the asymptotic suction boundary layer, revealing different vortex structures and their roles in the transition to turbulence, with one solution supported by a secondary shear gradient.

Contribution

It introduces two new localized travelling wave solutions in the asymptotic suction boundary layer, extending the understanding of flow structures related to turbulence transition.

Findings

One solution has near-wall vortical structures similar to known edge states.

The second solution's vortical structures are located in the free stream, supported by a secondary shear gradient.

The free-stream solution shows turbulence can originate from structures away from the boundary layer.

Abstract

We present two spanwise-localized travelling wave solutions in the asymptotic suction boundary layer, obtained by continuation of solutions of plane Couette flow. One of the solutions has the vortical structures located close to the wall, similar to spanwise-localized edge states previously found for this system. The vortical structures of the second solution are located in the free stream far above the laminar boundary layer and are supported by a secondary shear gradient that is created by a large-scale low-speed streak. The dynamically relevant eigenmodes of this solution are concentrated in the free stream, and the departure into turbulence from this solution evolves in the free stream towards the walls. For invariant solutions in free-stream turbulence, this solution thus shows that that the source of energy of the vortical structures can be a dynamical structure of the solution itself, instead of the laminar boundary layer.