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Helical and compact Navier-Stokes solutions with finite dissipation

ORAL

Abstract

For several Navier-Stokes solutions with initial helicity, two types of vorticity moment convergence are observed. First is a sequence of ν-independent times tm that correspond with a set of scaled, volume-integrated vorticity moments ν1/4OVm, with this hierarchy t ≤... ≤tm...t1 = tx and OVm = ∫Vl|ω|2mdV)1/2m. For the volume-integrated enstrophy Z(t), convergence of √νZ(t) = (ν1/4OV1(t))2 at tx = t1 marks the end of reconnection scaling. These temporal convergences form as localized knots shed double vortex sheets for these configurations: trefoil vortex knots, nested rings, orthogonal vortices and even initially smooth Taylor-Green. The second convergence develops for t>tx as reconnection and the growth of enstrophy Z accelerates towards approximate finite-time ν-independent convergence of the energy dissipation rate ϵ(t) = νZ(t) at te ∼1.5−2tx. Most strongly if the initial condition is compact and the computational domain can grow as the viscosity decreases. For perturbed trefoil vortex knots this is sustained over a finite temporal span ∆Tε giving Reynolds number independent finite-time, temporally integrated dissipation, ∆Eε = ∫∆Tε ϵdt, thus satisfying one definition for a dissipation anomaly with enstrophy spectra that are consistent. A critical factor in achieving these temporal convergences is how the computational domain Vl = (2ℓπ)3 is increased as ℓ∼ν1/4, for ℓ= 2 to 12, as ν decreases. Compatibility with (2π)3 mathematics is shown where ν≡0 Euler bounds small ν Navier-Stokes.

Publication: Kerr, R.M. 2023 Sensitivity of trefoil vortex knot reconnection to the initial vorticity profile. Phys. Rev Fluids 8, 074701.<br>Kerr, R.M. 2025 Compact Navier-Stokes trefoils in large domains with finite dissipation. In press J. Fluid Mech.

Presenters

  • Robert M Kerr

    University of Warwick

Authors

  • Robert M Kerr

    University of Warwick