Abstract
Viscosity, as a physical property of fluids, reflects an average effect over a chaotic microscopic motion described by Hamiltonian equations. It is proposed, as an example, that stationary states of an incompressible fluid subject to a constant force, can be described via several ensembles, in strict analogy with equilibrium Statistical Mechanics.
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Notes
In the subatomic world time reversal is not a symmetry but another more fundamental symmetry, CPT, could replace it in the following discussion
In the 3D case the equations are very similar (for instance the \(u_{\textbf{k}}\) are replaced by vectors orthogonal to \({\textbf{k}}\), ..): the main and important difference is that the expression of \(\alpha ({\textbf{u}})\) receives a contribution from the quadratic transport term which, although present also in 2D, cancels from \(\alpha \) essentially because in 2D when \(\nu =0\) the \({{\mathcal {D}}}({\textbf{u}})\) is conserved.
Hence depend on finitely many Fourier’s harmonics of \({\textbf{u}}\), i.e. are “large scale” observables”, but unlike the forcing, which only has the harmonics \(|{\textbf{k}}|<k_{max}\) with \(k_{max}\) fixed, no limit is set on the size scale
In many cases no “intermittency” is expected, i.e. \(n^N_\nu =m^N_D=1\).
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This is a redacted and updated version of a talk at the DinAmici meeting on 21/Dec/2018 at the Accademia dei Lincei, Roma.
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Gallavotti, G. Reversibility, irreversibility, friction and nonequilibrium ensembles in N–S equations. Boll Unione Mat Ital 16, 351–361 (2023). https://doi.org/10.1007/s40574-022-00343-7
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DOI: https://doi.org/10.1007/s40574-022-00343-7