English

Navier-Stokes bounds and scaling for compact trefoils in $(2\ell\pi)^3$ domains

Fluid Dynamics 2025-02-26 v3

Abstract

For a perturbed trefoil vortex knot evolving under the Navier-Stokes equations, a sequence of ν\nu-independent times tmt_m are identified corresponding to a set of scaled, volume-integrated vorticity moments ν1/4OV1\nu^{1/4}{\it O}_{V1} with this hierarchy ttmt1=tx40t_\infty\le\dots\le t_m\dots t_1=t_x\approx40 and OVm=(Vω2mdV)1/2m{\it O}_{Vm}=(\int_{V\ell}|\omega|^{2m}dV)^{1/2m}. For Z(t)=OV12(t)Z(t)={\it O}^2_{V1}(t) the volume-integrated enstrophy, convergence of νZ(t)\sqrt{\nu}Z(t) at tx=t1t_x=t_1 marks the end of the reconnection scaling phase. Physically, reconnection follows from the formation of a double vortex sheet, then a knot, which splits into spirals. ZZ then accelerates, leading to approximate finite-time ν\nu-independent convergence of the energy dissipation rate ϵ(t)=νZ(t)\epsilon(t)=\nu Z(t) at tϵ2txt_\epsilon\sim 2t_x and sustained over a finite span ΔTϵ0.5tϵ\Delta T_\epsilon\searrow 0.5 t_\epsilon, giving Reynolds number independent finite-time, dissipation, ΔEϵ=ΔTϵϵdt\Delta E_\epsilon=\int_{\Delta T_\epsilon}\epsilon dt, and thus satisfying one definition for a {\it dissipation anomaly}. Evidence for transient Kolmogorov-like enstrophy spectra is found over ΔTϵ{\Delta T_\epsilon}. A critical factor in achieving these temporal convergence laws is how the domain V=(2π)3V_\ell=(2\ell\pi)^3 is increased as ν1/4\ell\sim\nu^{-1/4}, for =2\ell=2 to 6, then to =12\ell=12, as ν\nu decreases. (2π)3(2\ell\pi)^3 domain compatibility with established (2π)3(2\pi)^3 mathematics in appendix allows small ν\nu Navier-Stokes solutions. Two spans of ν\nu are considered. Over the first factor of 25 decrease in ν\nu, all of the ν1/4OVm(t)\nu^{1/4}{\it O}_{Vm}(t) converge to their respective tmt_m. For the next factor of 5 decrease in ν\nu, \ell is increased to =12\ell=12, there is only convergence of ν1/4ΩV(t)\nu^{1/4}\Omega_{V\infty}(t) to tt_\infty and later νZ(t)\sqrt{\nu}Z(t) convergence at t1=txt_1=t_x and ϵ(t)\epsilon(t) over ttϵt\sim t_\epsilon.

Keywords

Cite

@article{arxiv.2401.03578,
  title  = {Navier-Stokes bounds and scaling for compact trefoils in $(2\ell\pi)^3$ domains},
  author = {Robert M. Kerr},
  journal= {arXiv preprint arXiv:2401.03578},
  year   = {2025}
}

Comments

21 pages, 9 frames in 7 figures