Clebsch Confinement and Instantons in Turbulence

Alexander Migdal

doi:10.1142/S0217751X20300185

paper

Clebsch Confinement and Instantons in Turbulence

arXiv:2007.12468 · doi:10.1142/S0217751X20300185

Abstract

We introduce a concept of Clebsch confinement related to unbroken gauge invariance and study Clebsch instantons: singular vorticity sheets with nontrivial helicity. This is realization of the "Instantons and intermittency" program we started back in the 90ties\cite{FKLM}. These singular solutions are involved in enhancing infinitesimal random forces at remote boundary leading to critical phenomena. In the Euler equation vorticity is concentrated along the random self-avoiding surface, with tangent components proportional to the delta function of normal distance. Viscosity in Navier-Stokes equation smears this delta function to the Gaussian with width $h \propto ν^{\nicefrac{3}{5}}$ at $ν\ra 0$ with fixed energy flow. These instantons dominate the enstrophy in dissipation as well as the PDF for velocity circulation $Γ_{C}$ around fixed loop $C$ in space. At large loops, the resulting symmetric exponential distribution perfectly fits the numerical simulations\cite{IBS20} including pre-exponential factor $1/ ∣Γ∣$ . At small loops, we advocate relation of resulting random self-avoiding surface theory with multi-fractal scaling laws observed in numerical simulations. These laws are explained as a result of fluctuating internal metric (Liouville field). The curve of anomalous dimensions $ζ (n)$ can be fitted at small $n$ to the parabola, coming from the Liouville theory with two parameters $α, Q$ . At large $n$ the ratios of the subsequent moments in our theory grow linearly with the size of the loop, which corresponds to finite value of $ζ (\infty)$ in agreement with DNS.

arXiv commentInvited review article for Int. J. of Mod. Phys. A. 67 pages, 10 figures, 1 table

Clebsch Confinement and Instantons in Turbulence

Abstract

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