$\lambda$-Navier-Stokes turbulence

TL;DR

This paper numerically investigates a modified Navier-Stokes model with a parameter λ, revealing a critical point where energy behavior changes, energy diverges, and turbulence intermittency reduces, offering insights into turbulence dynamics.

Contribution

It introduces a λ-dependent Navier-Stokes model, identifies a critical λ where energy cascade direction changes, and analyzes turbulence behavior near this critical point.

Findings

Kinetic energy diverges as λ approaches λ_c with a (λ-λ_c)^{-2/3} scaling.

Energy spectrum shows increased bottleneck effect near λ_c.

Intermittency decreases as λ approaches λ_c, indicating more regular turbulence.

Abstract

We investigate numerically the model proposed in Sahoo et al [Phys. Rev. Lett. 118, 164501, (2017)] where a parameter $\lambda$ is introduced in the Navier-Stokes equations such that the weight of homochiral to heterochiral interactions is varied while preserving all original scaling symmetries and inviscid invariants. Decreasing the value of $\lambda$ leads to a change in the direction of the energy cascade at a critical value $\lambda_c \sim 0.3$. In this work, we perform numerical simulations at varying $\lambda$ in the forward energy cascade range and at changing the Reynolds number $\mathrm{Re}$. We show that for a fixed injection rate, as $\lambda \to \lambda_c$, the kinetic energy diverges with a scaling law $\mathcal{E} \propto (\lambda-\lambda_c)^{-2/3}$. The energy spectrum is shown to display a larger bottleneck as $\lambda$ is decreased. The forward heterochiral flux and the inverse homochiral flux both increase in amplitude as $\lambda_c$ is approached while keeping their difference fixed and equal to the injection rate. As a result, very close to $\lambda_c$ a stationary state is reached where the two opposite fluxes are of much higher amplitude than the mean flux and large fluctuations are observed. Furthermore, we show that intermittency as $\lambda_c$ is approached is reduced. The possibility of obtaining a statistical description of regular Navier-Stokes turbulence as an expansion around this newly found critical point is discussed.