Subcritical transition to turbulence in quasi-two-dimensional shear flows

TL;DR

This paper demonstrates a subcritical transition to turbulence in quasi-two-dimensional shear flows, revealing a novel 2D mechanism driven by nonlinear Tollmien--Schlichting waves, which differs from the 3D case and has implications for flow control.

Contribution

It uncovers a new subcritical transition mechanism in quasi-2D flows, distinct from 3D turbulence, supported by stability analysis and numerical simulations.

Findings

Identification of a 2D turbulence transition mechanism

Demonstration of subcritical transition via nonlinear Tollmien--Schlichting waves

Implications for controlling turbulence in rotating and fusion devices

Abstract

The transition to turbulence in conduits is among the longest-standing problems in fluid mechanics. Challenges in producing or saving energy hinge on understanding promotion or suppression of turbulence. While a global picture based on an intrinsically 3D subcritical mechanism is emerging for 3D turbulence, subcritical turbulence is yet to even be observed when flows approach two dimensions, e.g. under intense rotation or magnetic fields. Here, stability analysis and direct numerical simulations demonstrate a subcritical quasi-2D transition from laminar flow to turbulence, via a radically different 2D mechanism to the 3D case, driven by nonlinear Tollmien--Schlichting waves. This alternative scenario calls for a new line of thought on the transition to turbulence and should inspire new strategies to control transition in rotating devices and nuclear fusion reactor blankets.