Spontaneous suppression of inverse energy cascade in instability-driven 2D turbulence

TL;DR

This study uses direct numerical simulations to explore how increasing instability-driven forcing in 2D turbulence causes a transition from vortex condensates to a state with suppressed inverse energy cascade and unidirectional vortices.

Contribution

It reveals the spontaneous suppression of inverse energy cascade and the emergence of shielded vortices in 2D turbulence driven by localized instability, highlighting the impact of forcing mechanisms.

Findings

Formation of large-scale vortex condensate at low instability forcing

Emergence of shielded vortices within the condensate at intermediate forcing

Suppression of inverse energy cascade and unidirectional vortex dominance at high forcing

Abstract

Instabilities of fluid flows often generate turbulence. Using extensive direct numerical simulations, we study two-dimensional turbulence driven by a wavenumber-localised instability superposed on stochastic forcing, in contrast to previous studies of state-independent forcing. As the contribution of the instability forcing, measured by a parameter $\gamma$, increases, the system undergoes two transitions. For $\gamma$ below a first threshold, a regular large-scale vortex condensate forms. Above this threshold, shielded vortices (SVs) emerge within the condensate. At a second, larger value of $\gamma$, the condensate breaks down, and a gas of weakly interacting vortices with broken symmetry spontaneously emerges, characterised by preponderance of vortices of one sign only and suppressed inverse energy cascade. The latter transition is shown to depend on the damping mechanism. The number density of SVs in the broken symmetry state slowly increases via a random nucleation process. Bistability is observed between the condensate and mixed SV-condensate states. Our findings provide new evidence for a strong dependence of two-dimensional turbulence phenomenology on the forcing.