Length-scale estimates for the LANS-alpha equations in terms of the Reynolds number

Abstract

Foias, Holm & Titi \cite{FHT2} have settled the problem of existence and uniqueness for the 3D \lans equations on periodic box $[0,L{]}^3$. There still remains the problem, first introduced by Doering and Foias \cite{DF} for the Navier-Stokes equations, of obtaining estimates in terms of the Reynolds number $\Rey$, whose character depends on the fluid response, as opposed to the Grashof number, whose character depends on the forcing. $\Rey$ is defined as $\Rey = U\ell/ν$ where $U$ is a bounded spatio-temporally averaged Navier-Stokes velocity field and $\ell$ the characteristic scale of the forcing. It is found that the inverse Kolmogorov length is estimated by $\ellλ_{k}^{-1} \leq c (\ell/α)^{1/4}\Rey^{5/8}$. Moreover, the estimate of Foias, Holm & Titi for the fractal dimension of the global attractor, in terms of $\Rey$, comes out to be $$ d_{F}(\mathcal{A}) \leq c \frac{V_αV_{\ell}^{1/2}}{(L^{2}λ_{1})^{9/8}} \Rey^{9/4} $$ where $V_{\alpha }=(L/(\ell \alpha {)}^{1/2}{)}^3$ and $V_{\ell }=(L/\ell {)}^3$. It is also shown that there exists a series of time-averaged inverse squared length scales whose members, $⟨{\kappa }_{n,0}^2⟩$, %, are related to the $2n$th-moments of the energy spectrum when $\alpha \to 0$. are estimated as $(n\ge 1)$ $$ \ell^{2}\left<κ_{n,0}^2\right> \leq c_{n,α}V_α^{\frac{n-1}{n}} \Rey^{{11/4} - \frac{7}{4n}}(\ln\Rey)^{\frac{1}{n}} + c_{1}\Rey(\ln\Rey) . $$ The upper bound on the first member of the hierarchy $⟨{\kappa }_{1,0}^2⟩$ coincides with the inverse squared Taylor micro-scale to within log-corrections.

16 pages, no figures, final version accepted for Physica D

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