Wall turbulence without modal instability of the streaks

TL;DR

This paper demonstrates through numerical experiments that wall turbulence can be sustained without the modal instabilities traditionally thought to be essential, challenging existing theories on turbulence mechanisms.

Contribution

It introduces a modified Navier--Stokes framework that suppresses streak modal instabilities, showing turbulence persistence without these instabilities.

Findings

Turbulence persists without streak modal instabilities.

Linear processes can sustain turbulence independently.

Challenging the necessity of modal instability in turbulence models.

Abstract

Despite the nonlinear nature of wall turbulence, there is evidence that the mechanism underlying the energy transfer from the mean flow to the turbulent fluctuations can be ascribed to linear processes. One of the most acclaimed linear instabilities for this energy transfer is the modal growth of perturbations with respect to the streamwise-averaged flow (or streaks). Here, we devise a numerical experiment in which the Navier--Stokes equations are sensibly modified to suppress these modal instabilities. Our results demonstrate that wall turbulence is sustained with realistic mean and fluctuating velocities despite the absence of streak instabilities.