Small scale exact coherent structures at large Reynolds numbers in plane Couette flow

TL;DR

This paper identifies families of exact coherent structures in plane Couette flow that scale to smaller sizes with increasing Reynolds number, providing insights into their role in boundary layer formation and turbulence transition.

Contribution

It introduces new families of scale-invariant exact coherent states in plane Couette flow, elucidating their localization, bifurcations, and potential contribution to turbulence.

Findings

Scaling ECS exist on smaller scales at higher Reynolds numbers.

Localized ECS near walls resemble attached eddies.

These ECS may influence boundary layer profiles.

Abstract

The transition to turbulence in plane Couette flow and several other shear flows is connected with saddle node bifurcations in which fully 3-d, nonlinear solutions, so-called exact coherent states (ECS), to the Navier-Stokes equation appear. As the Reynolds number increases, the states undergo secondary bifurcations and their time-evolution becomes increasingly more complex. Their spatial complexity, in contrast, remains limited so that these states cannot contribute to the spatial complexity and cascade to smaller scales expected for higher Reynolds numbers. We here present families of scaling ECS that exist on ever smaller scales as the Reynolds number is increased. We focus in particular on two such families for plane Couette flow, one centered near the midplane and the other close to a wall. We discuss their scaling and localization properties and the bifurcation diagrams. All solutions are localized in the wall-normal direction. In the spanwise and downstream direction, they are either periodic or localized as well. The family of scaling ECS localized near a wall is reminiscent of attached eddies, and indicates how self-similar ECS can contribute to the formation of boundary layer profiles.