$λ$-Navier-Stokes turbulence

We investigate numerically the model proposed in Sahoo et al [Phys. Rev. Lett. 118, 164501, (2017)] where a parameter $λ$ 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 $λ$ leads to a change in the direction of the energy cascade at a critical value $λ_c \sim 0.3$. In this work, we perform numerical simulations at varying $λ$ in the forward energy cascade range and at changing the Reynolds number $\mathrm{Re}$. We show that for a fixed injection rate, as $λ\to λ_c$, the kinetic energy diverges with a scaling law $\mathcal{E} \propto (λ-λ_c)^{-2/3}$. The energy spectrum is shown to display a larger bottleneck as $λ$ is decreased. The forward heterochiral flux and the inverse homochiral flux both increase in amplitude as $λ_c$ is approached while keeping their difference fixed and equal to the injection rate. As a result, very close to $λ_c$ 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 $λ_c$ 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.

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