On some properties of the curl operator and their consequences for the Navier-Stokes system
Résumé
We investigate some geometric properties of the $\operatorname{curl}$ operator, based on its diagonalization
and its expression as a non-local symmetry of the pseudo-derivative $(-\Delta)^{1/2}$ among divergence-free vector fields
with finite energy. In this context, we introduce the notion of spin-definite fields, i.e. eigenvectors
of $(-\Delta)^{-1/2}\operatorname{curl}$.
The two spin-definite components of a general 3D incompressible flow untangle the right-handed motion from the left-handed one.
Having observed that the non-linearity of Navier-Stokes has the structure of a cross-product
and its weak (distributional) form is a determinant that involves the vorticity, the velocity and a test function,
we revisit the conservation of energy and the balance of helicity in a geometrical fashion. We show that in the case
of a finite-time blow-up, both spin-definite components of the flow will explose simultaneously and with equal rates,
i.e. singularities in 3D are the result of a conflict of spin, which is impossible in the poorer geometry of 2D flows.
We investigate the role of the local and non-local determinants \[ \int_0^T\int_{\mathbb{R}^3}\det(\operatorname{curl} u, u, (-\Delta)^{\theta} u) \]
and their spin-definite counterparts, which drive the enstrophy and, more generally, are responsible for
the regularity of the flow and the emergence of singularities or quasi-singularities.
As such, they are at the core of turbulence phenomena.
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